Maybe some Summit County locals will chime in, but here's a start:
Try searching VRBO for the size of condo you want and locations, for example, here's a link to Keystone properties on VRBO: http://www.vrbo.com/vacation-rentals/usa/colorado/northwest/keystone
Keystone is not a bad place to start your accommodations search. I assume you are buying an epicpass good at Keystone, Breck, Arapahoe Basin. It would be a central location very close to Arapahoe Basin and also not too far from Breck, Vail and Beaver Creek. I stayed once at the Springs Complex of condos at Keystone and had a fine family trip: http://www.dcski.com/articles/view_article.php?article_id=1022
Again, assuming your focus is on resorts covered by the epicpass, since you have 3 weeks, perhaps you would like to relocate to Breckenridge for some of your time, it has a village at the base that is as big or bigger than Keystone's. And if it works for you, you could also stay at Vail or BC.
Meanwhile you can also go to central reservations at Keystone and/or Breck and see what they can do for you there: http://www.keystoneresort.com/plan-a-vacation/book-a-vacation.aspx