Is it possible that the NGP could outsell the 3DS? Yes ...
Is it likely? No ...
There are reasons why the successor to a market leading console has never been outsold by systems that are released after them. If a console can achieve strong sales during their time alone in the market, they can rapidly develop the userbase lead, library advantage, and the price advantage that attracts greater third party support and justifies the stronger sales later in its lifetime. To make matters worse, if the sales on the later console would be lower anyways (for some reason) this is likely going to be dramatically worse.
To demonstrate my point, suppose you have two systems of equal desireability and the same support that launch a year apart
Year 0 - System 1 0%, System 2 0%
Year 1 - System 1 100%, System 2 0%
Year 2 - System 1 66%, System 2 33%
Year 3 - System 1 60%, System 2 40%
Year 4 - System 1 57%, System 2 43%
Year 5 - System 1 55%, System 2 45%
(the percentage indicates both sales percentage and game library percentage)
Assuming constant support, it takes 5 years for the second system to make the competition close. Now, where things become problematic is that the earlier system's manufacturer will try to "hog" all of the development resources by trying to use their userbase advantage to ensure that most games are either exclusive to their system or at least released for their system; and if they're successful they may be able to sell a lot more than their competition simply because they have more/better games.
