In TTW all weapons that exist in both games (Fallout 3 and Fallout New Vegas) are the ones from Fallout New Vegas, so that includes energy weapons too.
I don't know if they are rare in Fallout 3 or not, I mean I remember finding a couple laser pistols right after getting out of Vault 101 in the Superduper Mart (not very good condition IIRC), then there is the Enclave later in the game and they use a lot of energy weapons, Mothership Zeta also has many energy weapons although they don't have iron sights yet (we are working on it), so a bit later in the Fallout 3 part of the game you will be swimming in energy weapons and if you are lucky you can find the Firelance quite early (but it has limited ammo and you need to be lucky to get the special encounter), it might be possible to buy some energy weapons early from weapon merchants but usually laser pistols and weak weapons like that but I can't remember very well (the last times I have been testing TTW I only used unarmed and melee weapons so I can't remember about energy weapons). Also some higher level Super Mutants (overlords for example) usually carry strong energy weapons but again that is later in game.
In Fallout New Vegas energy weapons have a target DT reduction with each shot (-5 DT IIRC) and that is the same in TTW even with energy weapons from Fallout 3.
Hope this helps with your question.