I don't think you'll have any problems keeping the three of those together, but you may experience some problems keeping the powder blue healthy. I have 2 pacific blues and 2 yellows in my 125 and have never had any problems with them. Tangs usually seem to fight amongst fish that are their own shape. The shapes in the 3 fish you mentioned are all dissimilar. A yellow will fight with another yellow (I don't know why mine get along so well, they're inseperable), a black, a scopus, etc. I've never known the pacific blues to be much of a threat to each other or other tangs their own shape, they'll square off once in a while, but it is never more than a nip here or a showing of how dominant they are. I don't know that powder blues do really well in a reef environment since they seem to be more of an open water tang than one that darts in and out of the rock like the pacific blues or the yellows, which kind of hover around. Powder blues are normally in large roaming schools on the open reef, and most reef tanks don't have the "open" reef they seem to like. If you can keep it happy, great, they are incredibly beautiful fish, but like I said, I don't think you'll have problems with them fighting each other any more than you will with any other kind of fish. Just don't add an achilles or a powder brown, or a gold rim or something with the same shape, or even a kole, I think that they would fight each other, although again, the koles occupy a different niche in the reef ecosystem than the powder blue, so despite similar body shape, they might get along with a lot of posturing.
