It's not as commonly done because it's considerably more expensive to build triple platforms, and the bends required to make it work slow the train down and cause wear. It's good for high-capacity stations though.
I get that it's more expensive, but then train spend a lot less time in stations for people to load/unload.
They don't usually have to be that sharp, but they tend to be noticeably sharper than the turns in the route.