We will be going to Belgium for two weeks and will be taking some day trips by train. Would you reccomend first class train tickets or coach tickets. Most trips are under three hours. Also can you reserve a seat with a coach ticket? Thanks!

Actually there is first and second class (not coach like an airplane). It's not necessary to pay more for first class tickets unless you want a better type of a seat and a small meal, things like those. Since the trip is under three hours then to me it's really not necessary.

Yes, you can reserve a seat in second class.

You can't reserve seats on any trains running within Belgium. Second class is good enough for most people. There is no point in buying any tickets until you go to catch a train in Belgium so you can decide then - you can even look at a train before buying a ticket.

You can reserve seats on certain international trains that take reservations (ICE, Thalys) with stops in Belgium (a Thalys ride from Brussels to Liege, for example, has mandatory seat assignments). On regular Belgian trains, no reservations; ditto for non Thalys trains running to the Netherlands and nonThalys/non ICE trains serving Germany. You don't get served a meal at your seat on first class regular trains (nor do you get one on ICE trains in Belgium, only on the Thalys).
As to whether you want first class, I usually travel first class, esp. at rush hour or on certain routes (for example, second class on weekend trains to Brugge can get VERY crowded and noisy).
First class is worth it to me, because train prices are very reasonable within Belgium, esp. on week-ends, it's not a huge difference to upgrade.
Whether it's worth it to you is something we can't say. My general rule of thumb is if a first class seat means I have to cut back elsewhere (meals, shopping, hotels, cultural events), then it's not worth it. If it has no or only minimal effect on my overall budget, then I will go first class. And if someone else is picking up the tab, then it's a no-brainer: first class of course!

Trains are a very popular means of commuting in Belgium. First class tickets might give you a slight advantage when travelling during these hours to find a place.

I'am sure that most trips will be under 3 hours. In 3 hours of effective train ride, you are way out of the country!

There is no need for first class tickets. The only train that I used (was there for 2 months) that was ever crowded was from Brussels to the North Sea, during rush hour on a Friday evening. If you are travelling within Belgium you will find that you can just buy a ticket at the gare and hop on, no need for reservations.