Knowing that AlphaZero beat Stockfish 8 (28 wins, 0 losses, and 72 draws), what would AlphaZero's rating be? I found three different questions here on SEC that asked what Stockfish 8's rating was for the match against AlphaZero, but none that asked what AlphaZero's rating should have been after (if it had been in a rated event, and assuming that it previously had an established, not provisional, rating).

Stockfish 8 was rated 3378 at the time per the official rating list. There was some dispute that it was handicapped, but since we cannot really factor that in accurately, it is better to ignore that for now.
Here's an online calculator for this. 28 wins, 0 losses, and 72 draws is an elo difference of exactly 100. 

Do remember that this score +28 =72 -0 score was against a handicapped version of Stockfish. In the full paper, AlphaZero scored +155 =839 -6, which is an elo difference of 52.