I just did this this morning. I checked my bag at the UA ticket counter in T3, and ended up going through security there. To use PreCheck in T1 would required shuttle bus ride to T1.
Granted it was 4:15am and I was the first premier to use the kiosk to print my bag tag, it only took about 20 minutes from standing in line/waiting for agent to call my name to tag my bag, to be thru security.
Then was walk to tram and quick tram ride to D gates - went quick.