Normally I would have recommended a Eurail Select Pass valid in Spain, France and Italy but France as of 2013 has opted out of that pass so you either have to go the Global Eurail route or do individual tickets - if doing that you can score discounted tickets on the individual railway web sites - those are sold in limited numbers - full fare tickets can be really expensive so then the pass may be better - and if you want flexibility to chose trains as you go along then the pass will be a good deal as well.

But for Spain check out www.renfe.com for Spanish tickets and discounts - www.voyages-sncf.com for those involving France and www.trenitalia.com for Italy.

A railpass in all three countries means getting mandatory seat reservations - usually 3 euros in France but 10 euros in Italy and about that in Spain - adds to the cost of a pass.

And passes are not valid on the Thello night train and on that and the Elipsos train if you act far enough in advance you can get some really cheap fares - especially if you go into a multi-berth compartment.

But note that the online discounted tickets must be bought weeks in advance to guarantee as they are sold in limited numbers and they are usually non-changeable nor refundable.

So it seems your best bet is going the discounted ticket route - if you want flexibility to chose which trains to take as you go along then look at the Global Eurailpass (since France is no longer part of the Eurail Select Pass, unfortunately as that would have been a great pass for you.

For loads of great stuff on European trains and planning a rail trip I always spotlight these IMO fantastic sites - www.seat61.com (great info on discounted tickets); www.ricksteves.com and www.budgeteuropetravel.com.

Again your best bet is to book ahead of time on those sites - forget any railpass for your plans IMO unless you really demand flexibility.

another angle however is whether or not you want first class - and there are significant differences between classes IME of riding European trains for decades - lot bigger seats, often more empty seats and more room for stowing luggage - as the railpass if over 25 automatically comes in first class then compared to first class fares it would be a good deal perhaps - but you can also often get discounted first class fares online - so do your homework - and again railpasses are not valid on the Thello night train so that is not part of the equation with railpasses.

No pass at all for those rides. If you do your homework, go online and score reduced-fare tickets in advance of your travels, you'll pay less.

At your age, you won't care a bit about 1st class tickets; even at mine, 2nd class is always just fine and affords me an opportunity to meet and talk with other passengers, which to me is a perk far more important than a few more inches to store my (very small) luggage.

2nd class is always just fine>

Well yes 2nd class is perfectly OK but much less fine IME of riding zillions of trains for years than 1st class. Many people in France and other countries pay more to ride in first class and they do so for the following reasons:

Free Wii - often in first class but not usually in 2nd class where you may have to pay for it if available

Seats 25% bigger as there are 1/4 fewer seats in the same-size train car

Lots more empty seats - I base this on zillions of rides on trains not occasional rides

More room to easily stow luggage - I often put mine on empty seats but as there are fewer of those in 2nd class you often have to fend for room in overhead luggage racks.

And best of all - seats that are aisle seats and window seats - seats with one in a row - no bothering others to get out or being bothered by others to get out, etc. And for couples you can have two facing each other - this is very desirable to some folks.

Discounted tickets can be had in 1st class often too and sometimes if the allotment of discounted 2nd class tickets is sold out then you may be able to get a discounted 1st class ticket at little more than a full fare 2nd class - this is not uncommon so check both classes.

Again there is a substantial difference between classes - 2nd class is comfortable enough but usually IME chock full - 1st class is often not that full and that makes a major difference.

2nd class is always just fine and affords me an opportunity to meet and talk with other passengers, which to me is a perk far more important than a few more inches to store my (very small) luggage.>

Folks who have never ridden in first class may be surprised to know that you can also talk to first-class passengers and would realize that the average first-class passenger is more, much more likely to know English as they are all in all better educated because they are wealthier - nothing wrong IMO in talking to such types - of course those fluent in French will not find this a difference but I have as a rule folks in first class are much more likely to speak English - and believe me they will talk to you!

I have ridden in 1st class, and I'm not interested in talking to people in English, thank you.

Well if you were in Italy or Spain like the OP is going to you may love to speak to folks in English, right, unless of course you are fluent in Italian and Spanish? And BTW first-class passengers in France would rather speak to you in French so why would someone whose French is perfect would even want to speak in English?

I have found that the French especially want to speak French rather than English any ole time if possible - many many Americans come back from France and say that 'the French can speak English but refuse to' - that I believe is just a false impression as most of the many many French folk I know thru my ex-wife and son speak very little French even though they may well study it in school for years - converstaionally just can't do it - and this is why in 2nd class it is much harder to find someone who speaks English (even train conductors often only speak 'train-conductor English') - folks in first-class yes are often business types who have to speak English in their jobs and thus are much more likely to be able to speak English.

You might find cheap flights between Madrid and Barcelona (it's a very competitive route). Check www.skyscanner.com and www.whichbudget.com and also look for flights between Barcelona and Paris. Flights in Europe can be cheaper than some of the trains (especially long distance).

You can check out the overnight train from Paris to Venice on Thello. There are some early discount tickets, but it might be cheaper to fly. Thello might only sell these 90 days in advance (perhaps 120).

https://www.thello.com/

Check Easy Jet for flights departing Paris Orly to Venice. Be careful of check in times and luggage weights.

http://www.easyjet.com/en

For your Italy flights, book on Trenitalia. Be sure to register first. Purchase for the faster trains is allowed 120 days in advance. You can often find snag discounts for the fast trains as cheap as 9€ (check both classes as sometimes 1st can be as cheap as 2nd). Screen-by-screen directions on this link (scroll until you reach the registering online section).
http://www.roninrome.com/transportation/booking-on-the-trenitalia-website

When you are using Trenitalia, make sure you change the hour when you change the date or it will default to the time of day you are using the computer. If trying to book late at night, it can cause it to roll over the next day.

2nd class is more than fine on the fast trains in Italy (and most countries in Europe) and more roomy than coach class on a plane. Venice/Florence is only 2 hours and Florence/Venice is 85 minutes. If you would book 1st class flights on short hops like this, then you might want to book 1st class trains. Here is a picture of 2nd class on AV trains in Italy.

http://upload.wikimedia.org/wikipedia/commons/b/b9/Frecciarossainterno.jpg

>>>Free Wii - often in first class but not usually in 2nd class where you may have to pay for it if available<<<

I don't find this to be the case these days.

You might find cheap flights between Madrid and Barcelona (it's a very competitive route)>

the new high-speed train however only takes about 3 hours or so city center to city center and at www.renfe.com you can score discounted tickets - for air travel add in costs of getting to and from airports and timewise flying will take longer all told - waiting for planes, waiting for luggage, etc, than trains. And what do you come to Europe to see - tarmacs at airports and large cities or the lay of the land in between as well. If you could save a lot by flying then yes but if the train is as cheap or nearly so see some of the Spain in between Madrid and Barcelona (not that that it is that scenic IME - largely barren landscapes and new high-speed tracks skirt cities so you see little of them.

>>>the new high-speed train however only takes about 3 hours or so city center to city center and at www.renfe.com you can score discounted tickets<<<

How much are these tickets? Random dates in late spring show flights for $52 and both airports have public transport that is cheap (5€).

Some trains only take 2 h 30 minutes city center to city center - yes full price is an astounding 110 euros or so but renfe.com offers deep discounts which I do not know how to access but there have been many threads about it - probably not nearly as cheap as flying - I wonder why anyone take the train and there are about 2-3 many hours - must have competitive fares I would say or no one would take the train?