Runegate,
Your motherboard has a pair of memory controllers which allow it to control two sets of memory at one time. This makes the memory run far better than almost anything else you can do. This is "dual Channel"
But it requires you to have matched pairs and amounts of memory in each "channel". That means, you want 2 of anything - and they need to be the same. So 2 of the 2GB modules is good. Or 4 of them.
But 3 of them makes the dual Channel stop working. (usually) This means that your computer will run more slowly. Will it run more slowly than one with 4GBs of RAM - likely yes unless you're doing very RAM-intensive work that can benefit from 6 GBs of RAM.
Here's what I suggest. Either return the set of 3 you now have and get 1 or 2 sets of 2 - for 4GBs or 8GBs or RAM. Or buy an identical stick of the very same RAM you have now.
Your RAM running at 1066 will have very little bad effects on your system speed compared to it running in Single Channel mode. Have you fixed that problem yet?
Oh and of course, you need a 64-bit Operating system to make use of more than 4GBs of RAM anyway.