With standard, 3v3 ctf the tactics are rather simple.
Most of the maps have seperate routes, and the basic idea is to have all of them covered. The fastest route is called the main route, while the slowest one is called the alt route ( like low on laos or up on voland ). Hence most of the action will go down on main route, you'll need to apply the meanest pressure there: two guys have to kill off the enemy, get in the base, let hell break loose, get the flag and score. These two fellows should stick together, complimenting each other's playing style. Most common tactics here are dual-gunning ( two steyrs, two rugers ), less used is one big-hitting semi and one auto ( eg. steyr + de ). The alt guy usually needs to go with a one shot gun, as he needs to take out only one enemy, and then his job is to annoy the respawning members, snatch the flag and make a getaway.
The tricky part comes in when the enemy is just as tough as your team. The alt route player is essential to achieve victory: either the mainers stall the enemy by holding mid while he gets the flag, or he has to be fast enough to get in the base with his teammates, thus trapping the enemy in crossfire.
Then again there are less 'smart' maps, like Dd2 or Death2, where you just have to pwn everyone and victory relies more on individual player power than teamwork.