Your post assumes UC 1 and 2 had to stop selling at a point, which seems highly unlikely. There is an Uncharted Duology Pack (1-2) as well as a Trilogy pack for the series. If anyone would have wanted to get the series at some point, he had the option to get all three games and not just Uncharted 3. So I am sure that the final 21 million sales figure that we got this Summer also includes numbers for UC 1 and 2 that are higher than what we knew of them.
The dual pack was already released before UC3 & was counted towards the 8 million total before UC3 launched.
UC1 & 2 never charted in Europe since UC3 launched, meanwhile UC3 continued to chart for an entire year, sometimes at #1 MONTHS after release, hell it sold better than Mass Effect 3 on PS3 or 360 in Mass Effect's second week in some markets.
UC 1 (as of E3 2009) : 2.6 millon
UC 2 (as of March 2015): 6 million
UC 3 (as of March 2015): 6 million
Stop using wikipedia, it quotes the banned site.
This doesn't count Uncharted Golden Abyss, which sold close to 600k (retail as per creamsugar) in NPD. It might be higher or lower but I vaguely remember it atleast was above 500k. It sold around 100k+ in Japan. No idea of EU though.
The 21 million does not count the Vita title, it was specifically mentioned in the 21 million announcement that those figures were only for the titles included in the Nathan Drake collection.
Now lets assume UC 1 somehow didn't move a single unit since 2009 (unlikely) then we get a number of around 15-16 million for the series. I just don't see Uncharted 3 carrying a large percentage of sales here i.e more than 10 million.
You don't see it, but according to the PAL charts in a random week in May 2012 (7 months after launch):
http://www.neogaf.com/forum/showthread.php?t=474892
It's still in the top 10 in Germany (EDIT: Actually, top 5), you can sort through the PAL charts for most of 2012 to find it still charting, & yes UC1 & 2 were nowhere to be found.
The absolute bare minimum for Uncharted 3 is 7 million because that's the average of 21 million split between three titles, but since we know that it sold 5 million in its first month alone & it also doubled UC2 in its launch week, & we know that Uncharted 1 sold 3 million as of UC3's launch, then 9 million is the lowest estimate.