I'm late to this, but I would just say that 3mm is way more clearance than you need. Find a spindle that gives you maybe 1mm of clearance, or do the same thing with a Shimano (or equivalent) cartridge. If you go with a cartridge BB, start with a 115 mm and add up to 2 1/2mm of spacers under the drive-side cup to fine-tune the clearance. You could also experiment with replacing the standard 3.8mm spacers under the granny ring with 3mm spacers--that will gain you a millimeter. It's impossible to say for sure if that would effect your shifting onto and off of the granny ring, but there's a good chance that it will make no significant difference.

I would not dent the chainstay to gain clearance. That's a knuckleheaded Primitive Pete method if ever there was one. )

EDIT: Oops, did I just call someone a knucklehead? Sorry--no harm meant. I'm just prejudiced against dented frames.