It's all up to who will be able to boost local production in China the most ... In 213 and beyond. Also:
What's new for Audi in 2013? A boost from A3 & LCI Q5 + Q5 Made in China. That's it.
BMW: boost from 1er 3dr, partially a boost from 3er Touring, boost from 3er in Chinese market (it was absent there till Sept 2012), boost from X1 LCI, X1 in US & Chinese market, LCI X3 is coming, new X5 is coming, 5er LCI is coming, 3er GT is coming, 4er Coupe is coming, i3 as well.
MB: boost from A-class & CLA, boost from new GL, partially from new ML as well & GLK LCI, marginally still from C-class LCI, definitely boost from E-class LCI, and some boost from new S-class.
IMO BMW will retain no.1 position in 2013.
2014 will be more interesting.
With A3 sedan coming, new Q7 for Audi.
GLA & new C-class for MB, MLC, new CL.
For BMW: 4er Cabrio & 4er GC, Active Tourer, 2er coupe & cabrio, X4, new X6
So, beside some niche models (SACs, 4dr coupes) it will all come down to the fact who will the quickest @ downsizing: the expansion of A3, A/B & 1er lines. BMW are also planning to launch an A1 rival - no word from MB yet regarding this particular agenda. We can expect a minivan from Audi - to fight B-class & BMW's Active Tourer.
And then there is a production facility limitation ... who will be able to expand the production facilities on time. Especially in fast growing markets.
IMO Audi won't overtake BMW anytime soon. Except they totally boost Chinese production in no time, and BMW doesn't follow with same tempo.
MB has a good chance to overtake Audi in 2013 or in 2014. But it still comes to China where Audi is just too strong - Audi fate depends on China ... to successfully defend No.2 position, or even take No.1 position as best-selling luxury car brand worldwide. If Audi is much stronger then MB & BMW in China, anything can happen.