A 125g would be fine for quite a while but what we are trying to get across to you is that you WILL need a bigger tank than that if you aim to keep them for life.
Just as a guide my 12"+IT is like 6 to 7" tall and like 3" wide and hes got alot of growth still in him so imagine just 3 of those in a 125 never mind a fully grown threesome.
Good luck.