we'll be in italy during last week of august + first week of september (high season, eek!).

for train travel between rome - florence - venice, should i get the 1 country pass that allows me to travel 3 days?

if we decide to take day trips from let's say rome to pompeii or florence to siena, how do we book those trains?

You are not traveling enough to warrant even considering the Italy Railpass plus on every train you ride with a pass you pay 1`0 euros supplement (except regional and parts of some IC trains) - go to www.trenitalia.com for sample fares and there are so so many trains between those cities do not worry about getting on - you do not need to buy tickets before arriving in Italy but if you want to score some nice discounts book wqeeks ahead to get the limited in number discounted tickets that can be as cheap as 9 euros flat fare on high-speed trains such as those between those cities. But those tickets are train-specific and are hard to change but if you are sure of your dates book them now and have everything in order and just hop on the train.
If in Rome and wanting to go to Pompeii and back just buy a ticket to Naples Centrale then take the CircumVesuviana train from there to the Pompeii Scavi station (right at the entrance to the ruins) - takes about 3 hours all told each way from Rome so is a long day trip but you may be able to squeeze in a quick look at fascinating Naples to boot!
For lots of great info on Italian trains check out these IMO fantastic sites - www.seat61.com; www.ricksteves.com and http://www.budgeteuropetravel.com/id12.html.

Actually if you do the day trip to Pompeii plus two other rides you may take a look at the 3-day pass vs full fare tickets you would buy once there - discounted tickets would be cheaper than a pass but perhaps full fare would not - not sure.

Oh when researching the Rome to Pompeii trains keep in mind that you can only buy a Trenitalia ticket form Rome to Naples - then you have to change to the narrow-gaugue CircumVesuviana Railway - you just buy that few euro ticket after arrival in Naples - buy it at the entrance to the CircumVesuviana station (attached to Naples Centrale - just follow the new and improved walkways taking you there.

For a day trip from Florence to Siena, it is better to take the SITA bus, since the bus stops in the hill town of Siena while the train station is down the hill outside town.

If you Do take the train from Florence to Siena there is a city bus which will take you up the hill into the town or you can easily get a cab.

From Siena's train station there is also a new funicular or some such people mover I've read that now takes you to the hill top from the train station - but trains take longer because they, due to hilly terrain, must take a roundabout routing via Empoli - buses go straight thru the hills of Chianti to Siena and are also cheaper.

PalenQ - glad to hear the new walkways in Naples train station are finished. That was quite a walk when all the scaffolding and barricades were up!

Alifafa - Pompeii is a doable day trip from Rome but plan to leave early so you have enough time to see the ruins before they close.
I couldn't get my partner moving in the morning so we ended up on a 10am train to Naples. The walk from the local train to the entrance isn't long but he had to stop to eat!!! So 6 hours of traveling and we had about 2 hours there, grrr.

>>>From Siena's train station there is also a new funicular or some such people mover I've read that now takes you to the hill top from the train station<<<

There's a series of escalators, but you probably don't end up in the area you want to be. The bus (local city or the Sita bus from Florence) drops at a much more convenient place to most things than the escalators.

The bus is better in many ways - cheaper and right up to the action, almost - still a bit of a walk.