Must be a bad one if your looking at 12 weeks.
You'll regain 98-100% shoulder function, don't be all hasty and go back too soon, listen to the Dr's and do you rehab.
Good idea to just think about some bike tracks around the neighborhood rides first, then progress back into DH, the issue being you'll lose general fitness and strength. It's a shitty bone to break.
Broken mine a few times, not issue unless I have a massive solo session when the wife goes to bed, those bloody internal rotators needs more endurance.