I had a yellow and a purple in a 72-gallon for over a year, and they were always going at it, sort of. They had their own sides of the tank, but as soon as one crossed the line they were trying to tear each others tails with their barbs. Every so often, one or both would have their tail's split, which would heal shortly after, but I wasn't comfortable with the stress levels between them. IMO I would never do that again, even if I had a larger tank. A 150-gallon, might be acceptable for two different shaped tangs, but pick between either the purple, yellow, or sail fin instead of getting greedy. Remember, your charges are supposed to happy and comfortable because they are no longer in a home that has millions of gallons to swim in. Tangs need a lot of swimming room depending on the species, and since sail fins get larger than the yellow and purple, eventually one will become the dominant one. You can't always assume that just because they are introduced at the same time, they will always be "friends". And not ALL good cases are a sign that it can work for others. Most of the time tang incompatibility will force you to remove one eventually, assuming one doesn't die from stress or battle wounds first. A yellow and a regal should have no problems getting along, even though it seems like the clown tang is out of place since those get pretty large compared to the other two. A 65-gallon seems too small for three tangs.