Tacoma will not hold 3 carseats across the back. 1st gen tundra will if you have the right car seats (all 3 can't be those giant britax type seats), and I know this because my brother had to do this. One of his might have already been in a booster seat though.
I would imagine that a 2nd gen tundra definitely would fit 3 across. Get a nice shell for it.