I've only used T1 at LAS and D gates since I fly with AA or DL.
Why would they have D gates for both T1 and T3--couldn't that cause confusion for some pax.
All D gates exist in a satellite gate area. The D gates are accessible by tram from either T1 or T3.
T3 is newer, nicer, a shorter tram ride to the D gates, and less crowded.
You can access any D gate from T3. I fly AA and DL and don't use T1 to fly out of LAS unless I have a bag to check.