125 gallon could easily house three yellow tangs, but make sure you put them in at the same time and observe their behavior. Better to get baby ones so that they can grow up together, possibly averting some aggression, I have a sailfin and a yellow tang together with no problem; they have grown up together since they were babies from my LFS.
If they start to attack one another, like the other posts state, beware, but be aware. They can change their behavior quickly. But if you were to get three babies from a batch at your LFS, most likely you should have no problems.
Good Luck.