Thomson have introduced a new 'Select a seat' option where passengers pay a bit extra to choose their seats. However, Thomson usually preseat groups or a good handling agent should usually look at the seat plan and seat groups together.
Did you book it all in one go or seperately? Either way, turn up in good time and it should give you a bit of flexibility if there is a problem.#
PS. I don't know if you will be flying on a Thomson 767 or a ex-First Choice 767. If its the First choice the config is 2-3-2, so a middle 3 would be good for you as there is 3 of you? With the Thomson aircraft you could always have the 2 and then 1 across the aisle or 3 in the middle, your call!