Frankly, a beer this big may not be done at 3 weeks. It's been long enough at this point where I'd start raising the temp slowly to help get every point of attenuation that you can. Better to start now than to wait until the yeast has already given up the ghost. You'll have more luck waking up a sluggish but active yeast than one that is largely dormant already.
Bump it 2-3 degrees, wait a few days, and repeat until you hit the low-mid 70's. I wouldn't rack it until you're absolutely sure it's done in primary. I let my big beers sit in primary for at least 5-6 weeks before I rack/bottle. No harm in doing so that I've ever seen.
And in all honesty, even if this beer is truly finished this high, just be patient with it. For a beer this size, it could still be a real nice sipper after a year or so even if it finishes on the sweet side.