Harold, I mostly like CAST's Universal Design.

Multiple means of representation follows from Spiro's Cognitive Flexibility theory (great likelihood of comprehension, activation for a relevant problem, and matching a framework to solution).

Multiple means of engagement: tapping into Keller's ARCS model (and my own work on engagement).

My only quibble is with multiple means of expression: that depends on the degree of transfer being sought. If it's specific, e.g. chemical hazard identification, I want one means of expression: correctly identifying the toxic material found, across multiple contexts but only one 'right answer'! If it's broad, e.g. 'justice', then I'm happy to have it expressed in multiple ways.