Unsure how to make this..please help!!!?

I'm making Bar-B-Q Beef sandwhiches. The receipe I'm using says to bake the roast at 350 degrees for 30 minutes, then bake at 275 degrees for several hours until it's done, and shreds easily with a fork. I want to make this in the crock pot how long would it take? It's a 3 pound roast..so approximately how many hours?!