I'd want at least a 6' tank, preferably 8'.
My experience with multiple tangs is that if you get them all small, you'll be OK. They may squabble a little but not anything too bad.
However, once they turn into adults and get real territorial, you'll have issues even in big tanks. It may never turn into fights to the death, but it also may. I've never had 2-3" tangs get so serious about fighting that one is killed, but I've had large tangs get there very fast and I've had to remove the one who is getting thrashed.
So that's my two cents... not only does tank size matter (get the biggest you possibly can) but also the starting size of the fish matters. Get young tangs and hopefully as they grow up together they wont cause issues.