Yeah Gen 3 has 3 mounting holes vs Gen1/2’s 4, which is the visual clue. Gen3 is the one you want anyway. Most rack refurbishers throw 1 & 2s in the bin. They were obsoleted years ago.
Yep, you will need the MK6 harness to go with it. It’s all covered in the gen 3 rack thread in how to guides & troubleshooting.