Asking which Unix (should I learn / use / buy ) is a bit like asking which car is the best: Everyone has their own opinion, and none of them are really right.

Most "free" distributions of Unix (Linux, BSD, etc...) have some sort of multi-boot loader avalable with them. Linux has several that work pretty well, once you get the config just right.

If you really want 3 different OS's, MY opinion would be Linux, FreeBSD, and Windows.

"Unix" as a product died years ago - now it's generally acceptable to call a Unix-like OS Unix, so technically Linux and FreeBSD are both Unix.

Have fun!