Guardiola in his last season 2011/12 was experimenting with a back 3 formation a lot -
Barcelona 2011-2012, but looking back i'm suspecting it didn't work because he didn't have the right players (mainly defenders) to make it work properly.
The good thing about this system as the game vs Milan showed is that the basics of their tried, tested & successful tiki-taka style won't be compromised & it potentially can make them better.