There's no such this as "dual channel" or "tri-channel" memory - the difference lies in the system's memory controller. A system with dual memory channels can access two DIMM modules simultaneously in order to increase throughput - a tri-channel system can access three modules at the same time.
Memory is sold in kits of 2 or 3, but it's the same memory (all other specs being equal). Personally I'd just put all three sticks in your system. The third system won't have a mate to pair up with so it won't have the same throughput, but the difference in performance is very negligible in real terms and if the extra memory helps avoid some disk accesses then you'll be much better off with it than without it.
Just be sure to consult your motherboard manual to make sure which DIMM sockets to use.
My experience is that is incorrect. Very few motherboards will ignore an un-matched single module and allow the other matched pair to run dual-channel. They'll come up showing ONLY single-channel RAM and that's it. I've seen mention in two manuals, I believe high-end Intels, that they would attempt to address some as dual and add 3rd (or unmatched 4th) to the one of the channels - but it was very iffy. Most say 3 modules and no dual channel at all.
Yes, Runegate, you can either add one module or remove one to get dual channel. As said, it's not the memory but the motherboard that has the dual channel ability. The only requirement for the memory is that it be in matched pairs.