Not sure about ZenithCS's claim of duel channel over 3 dimms (sticks of memory), but that would be very rare to the point where I doubt it to be true, and if it is true then it is very rare indeed. Most older systems had 3 slots as a holdover from the PC133 days, and if you wanted performance then you would use 2 in duel channel, but if you needed bulk space (at the time 768MB was considered HUGE) then you would run 3 in single.
Memory itself is single channel. There is no advanced logic on the memory sticks themselves to make them communicate with each other in order to work together. Everything is done through the north bridge (NB) (a chip that has been integrated into the processor on AMD systems for years, and is just now beginning to disappear on the Intel side). The NB is the computer that runs and manages your motherboard's fast and high priority devices (while the south bridge (SB) controls legacy and slower devices, and company specific features).
To run your system as duel channel you have to have matched pairs of memory. The timings have to be the same for all modules, and if they are not then the faster memory will be under-clocked in order to match unless you slightly overclock your ram through BIOS to compensate by manually specifying the timings (not hard to do, and most ram can be over clocked a little bit with no harm).
Some people claim to have huge performance hits if they do not buy ram kits to run their memory as multichannel, but I have never seen this to be true for myself. Perhaps it is merely a high end thing, or a DDR3 thing. But over the years I have built and worked on several DDR and DDR2 rigs and seen similar performance weather I bought a kit, or found single sticks for a very good price. I have even mixed and matched brands with no problem at all. So long as your timing and speed ratings match it should be fine. To those who think a kit is required; do you think the manufacturer is going to test each pairing for best performance? The chips come off a convayer belt, are tested for speed and timings ability, and then are put in their packaging. If all they are doing is matching timings and speed, you are perfectly capable of the same thing.
In short, you should buy a 4th stick of ram, or trade it in for 2 smaller sticks of ram (are you running a 64bit OS so you can see more than 3.5GB of memory?). But try it, you may not know the difference unless you are doing some real gaming or other performance oriented task.