After much fiddling on my now-defunct mtb with a 9 speed rear derailleur, I could reach all but 2 gears without rubbing with any ring. However, I never ran more than 4 gears on a given chainring, i.e. granny --> 1,2,3,4, middle ring --> 3,4,5,6,(7) big ring --> 6,7,8,9. If you cross more, it's not good for the chainline anyway. Since I roughly had a 1.5 gear overlap between rings with my choice of rings/cassette, I had 12 effective gears.

In case you're surpriced 12 "real" gears are less than the theorical 27 available, remember that all these gears are just marketing BS from Shimagnolo.