So I have two drives and have bought a third as a back up.

I'm looking for some simple/light software that makes sure that my third drive is an up-to-date copy of the other two. That sounds simple, but can't find anything.

Any recommendations?

Thanks!

I've found Nero Backitup to be pretty good, and may do what you need and IIRC it has an incremental backup feature. It's free, so probably worth a try if you haven't already done so.