Three sets of three rules each.
Opening:

  1. Control the center
  2. Develop pieces
  3. Castle! [1]

Midgame:

  1. Attack Weaknesses
  2. Improve Your Position
  3. Identify the Threat (and avoid mistakes!) [2]

Endgame:

  1. Activate the King
  2. Attack the pawns
  3. Get a passed pawn

Here be dragons:
Do not create spaces for your opponent that are not there in the first place.

Notes:
[1] This rule is best defined as “protect the king”, with some lines of thought arguing that castling may not always be the best move (!research). [2] Possibly the most important thing in chess).