Looks like one supports both DDR2 and DDR3 while the other two only support DDR2.
Looks like only one has two PCI Express 16x slots while the other two only have one.
Because of that, one board has only two PCI slots where the other two have three.
Two boards have single LAN ports while one has two.
There may also be some slight differences in the back pane in number of USB ports, Firewire and eSATA.
All three boards really are essentially the same but with the differences noted. You have to decide what you are going to do with your computer and check off the ones you want/need. Then choose accordingly.
I, for instance, didn't need SLI or CrossFire so only needed one PCI Express slot and I was not going to pay for DDR3, nor did I need two network ports so I went with the GA-EP45-UD3R.