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.

I believe United checkin is terminal 3 but gates are in D. I usually use terminal 3 when flying AA with no checked luggage since the tsa lines are usually shorter at T3 and T3 has less drop off traffic.