Firstly -- If you care about money at all, don't buy your memory from Dell.
They commonly have a 200-500% markup on their RAM.
www.newegg.com is usually a good place to order from and you can use their memory configurator to make it easy (and they guarantee compatibility or your money back)
As for triple channel memory...
Can you use something other than 3 or 6 modules at a time? I believe the answer is no. I really don't think so based upon how multi-channel RAM actually works. It would be *technically* possible if the board manufacturer *really* wanted to do it... But I firmly doubt
any board actually does support a [2+1, 2+1, 1+1+1] configuration in triple channel. I can't tell you no for sure though, because I've never seen anything that actually says you can't do it. I'd like to point out that some boards do support mixing sizes for dual channel, but it has to be (just as an example) [2+1, 2+1] or [1+2, 1+2]... The sizes need to match for each paired bank. But even under this consideration, you'd still need [2+1, 2+1, 2+1]. The config you proposed shouldn't work.
Something else to consider is the disappointing lack of a noticeable performance increase:
"Although three channels of DDR3 memory might seem excessive, the Core i7 really does seem to make good use of faster memory, at least in synthetic tests. But that was to be expected. The real question is whether those gains translate to real-world applications, and it's here that the results are more mixed. Certainly, in scientific computing tests like Euler3d and MyriMatch, which we already know to be sensitive to memory subsystem performance, faster memory can provide tangible performance perks.
However, common desktop applications like those highlighted by WorldBench don't benefit much from higher memory clocks or tighter timings. Neither do most games, which at best show minor frame rate improvements that aren't significant enough for most folks to even notice, let alone appreciate."
http://www.techreport.com/articles.x/15967/8
I'm all for souping up your computer, but moving to triple channel from dual channel is only useful in terms of bragging rights, really. If you're willing to drop some cash though, there are other ways of improving your system performance that will yield better gains. I wouldn't get too bent up over running *just* a dual channel memory config, though.
In all honesty, 3GB is enough memory to do pretty much anything you'd like at the moment. If triple channel is THAT important, then try it out [1, 1, 1] and see if you like it. If you think it runs better -- I mean, RAM *is* pretty cheap -- you could buy another 2GB module and then maybe you can swing [2+1, 2+1, 2+1]. Personally though, I don't find the results compelling enough to do it.