You only need to have one of each of the three T5 snipers in one game of CHIMPS. This does NOT mean that you need to beat the CHIMPS play (round 100). Simply having them is good enough. Note: I do not know if you need to beat at least one round for the game to register having all three or not.
Personal Experience: I had all three T5 snipers at ~ round 70. I failed on round 99 of 100 as I had alt-tabbed out as soon as I had all three T5 snipers active at once. When I returned to my PC, I simply returned to the main menu and noticed that the achievement popped on the reward screen in-game, and on Steam!