Terminal 3 technically refers to the newly built E gates at McCarran which are designed to handle large international flights and relieve pressure at the older Terminal 1.
Although the remote D gates are technically part of Terminal 1, if I'm flying in to the D gates, I find it so much faster and easier to leave the airport via the Terminal 3 E gates. Similarly, if your airline allows check-in at Terminal 3, I find that waits at both check-in and security are shorter here, and again it's a breeze heading towards Terminal D.
The E concourse itself is quite spacious and seems to have more basic amenities than the older A, B, and C gates. Gets the job of ferrying large amounts of tourists in and out done each day.