I remember messing with the GTS in Gen IV and trading between my HGSS and DPPt with only one DS to get starters from vice versa. If trading is any way like it was in that, then maybe you can do it with ONE 3ds and 2 carts...
The trick was this (sorry if you knew about this already):
What you do is you start a game on the cart you're going to play on, throw a "rattata" (for example, use any common mon) up on the GTS and ask for the starter you want in return. This is the style of trade where you lose your mon to the internet, ask for what you want in return, and if you're lucky you'll get what you asked for after awhile when you log on again to check. You do a rattata because NO ONE is going to give you a starter for your rattata, and if they do then the next step is moot.
Then you load up the other cart, do a search for "rattata" or people who are looking for the starter for a rattata, or the friend code, etc., but just look. If you refresh enough times, you'll find YOUR posting of the rattata, trade your starter for it, then repeat. You only have to do it twice to get all three starters.
It will probably work completely different (it may even already, ahven;t trade since genIV) so this post was probably just a waste of space, but who knows
http://www.youtube.com/watch?v=GtxCr5ju2EE&feature=related - If you played Mystical Ninja 64, you need to click this.