combined free speed of 3 motors

[Ether]

Worthy of note: If you combine motors with substantially mismatched free speeds, the free current for the combination can be large, and thus the value of Kt for the combination at high speeds will be less than the value calculated at stall.

I think this may be what James was trying to say.

edit: If you look at the last graph, you'll see that the 550 probably won't last long in this setup.