If it is a 4' tank, it is possibly big enough for 3 but you would be pushing the envelop. Others may say it's fine. These fish get pretty big though, probably 4" in a decently managed 75 gallon, move around a lot, and are a little aggressive. I'd still buy all females. Mine are all of 4" and except for a Melanurus Wrasse, rule the tank.
It's a 4' tank, and we already had 2 females who were lost when a purple tang infected with velvet wiped out our entire stock. We put them all in a hospital tank and treated with copper, but for the smaller fish it was too late, but it seems to have worked for the Tang which went back in just this past Sunday. We thought he was deceased, but when he went back into the unmedicated water, even though at first he was laying flat on the bottom, he's made a recovery that is nothing short of miraculous. He has his color back, and he's eating like a horse, but we're still keeping a close eye on him.
I'm unsure why he had a near-death experience, my best guess is that we simply had him in medicated water for too long. We caught it just in time, though, I think another 5 minutes and he wouldn't have recovered like he has. In fact, I thought he was dead and gone, but after about 10-15 minutes in the display, non-medicated tank, he was swimming around like nothing happened.
I plan to have a little chat with the owner of the store who sold me a diseased fish about replacing our stock, but haven't yet done it. But I'm disabled and unemployed, so if he gives me a hard time, I'll just drive me electric wheelchair up to his store and sit in the parking lot with a sign that says "This store sells diseased fish, go elsewhere.", if he refuses.
I'm absolutely heartbroken about it, but what's done is done and from all present appearances, at least we saved the Tang. He's beautiful, too.
And before the Tang Police issue a warrant for my arrest for putting a Tang in a 73, if he ever gets too big for the tank, we'll either get a bigger one, or sell him to someone else who has one, and buy a new smaller one.