Before doing anything else, buy yourself a brand new 3mm allen wrench and try using that to unscrew the bolt. You'll be surprised at how often that works. Then throw away your old one before it can round out any more 3mm bolts. Allen wrenches don't last forever.
If that doesn't work, go to a tool store and get a small lefthand drill bit and a replacement bolt. The lefthand drill bit will probably back out your old, rounded screw.
Put the new bolt until it just starts adjusting the brake caliper. Now take a 5mm allen wrench and adjust the caliper by eyeball until it looks pretty even. Make whatever fine adjustment you need with your new 3mm set screw and you'll be good-to-go.