My best advice to you is to have one or the other in such a small tank. However if you just have to do it...You can keep a 3" BTA in a 7.5G tank. It will however eventually outgrow the tank. As to keeping the BTA in the tank with other corals, that will be the trick. It depends on the other corals. If there are just a few and you can temporarily move them to a corner out of the way, you might get lucky and the BTA will settle in a spot. Turn off all flow to the tank, place the BTA where you would like it to stay and leave the flow off for 20 minutes. Then turn the flow back on. Then you can more the corals around staying outside of the BTAs reach. But of course, the BTA can move anytime it wants to and they frequently do so. Might not look too good but you can corral the BTA using egg crate to separate it from the other corals. If you chose to do this you need to be sure there is enough room for the BTA to expand fully without being cramped by the egg crate.
I know someone who has kept a BTA in such a small tank for more than 6 months and he only uses Tap Water in the tank. It was an experiment that he has just kept running. His BTA still looks healthy and happy! BTAs are a lot hardier than most would think.
Of course it sounds like an excellent excuse to upgrade to a larger tank! LOL