Interesting thought. However, you overlooked a few key elements.
3DS is a new system, with completely new games. Obviously it will be counted separately, just like the 360 is not lumped in with the original Xbox.
Your argument misses sombe key points, though. For one, you say that slimmed-down hardware still counts, and yet you say the DS Lite (which is literally the same as the DS, just smaller) should be counted separately.
Second, you say that due to the DSi's slightly-increased power it should be counted separately. There has not been a single retail game released exclusively for the DSi, and none have ever been announced. Sounds like the same system to me. You may try to counter this with the fact that, unlike previous DS', the DSi has downloadable games, and thus should be counted separately. I would then motion that all the Xbox 360s that were sold without a hard drive be separated from those with one, as the same situation applies.
Ergo, DS, DS Lite, and DSi all get counted together, while 3DS is obviously its own console.
