in my experience, 3 inches should be plenty far! i actually have mine about an inch away from anacropora and a large brain coral. as i don't have all my corals cemented in place, i had the hydno shift and got a little too close (almost touching), which caused the trouble. neither my brain nor my acropora have recoved in the "burn zone" yet.
otherwise, they are totally beautiful and fluoresce like nobody's business!
good luck!