The only way youre really going to know that is if you try each combination yourself and test them with memtest and sandra or other benchmarks. In my case, slots 1 and 3 and 2 and 3 gave me errors in memtest and I could not get my system to be 100% stable. In 1 and 2 they run flawlessly. Though I know others on here with the same board and same ram run them in different slots without errors either.
You should know that 1 and 2 are not dual channel. Though I have only seen a ~100 mb/s difference between single and dual, so for me single channel is fine.