i really think that you wont have enough room with 3 tangs in a 180. tangs are fish that cover MILES of ocean in a single day, so they need a lot of room in out tanks.
the hippo (hepatus), gets to be around 12 1/2" long
the yellow gets about 8" long
the powder blue gets about 10" long
that is a big load for a 180, IMHO.
the powder blue tang
is VERY suseptable to ICH, not to mention that a powder blue can get EXTREMELY hostile towards other tangs. especially if they are close in form and color (like the hippo is).again IMO, unless it is a very large system, a powder blue should be the only tang. AGAIN, these are just my opinions. HTH