These were HobbyKing Super Simple card programmable ESCs, not Turnigy Plush. They are virtually identical to the Towerpro 18A.
I don't think I2C is really needed. PWM can easily go up to 400Hz which is plenty fast enough.
This thread has a vast amount of Tricopter info. Tricopters use three motors for roll/pitch and rotate one motor for yaw. Stabilization can be done with one standard heli gyro per motor plus one for yaw. You use helicopter CCPM mixing to generate the correct throttle signals. This approach has a few drawbacks so I would like to have a go at doing all of the signal processing on-board.
And some notes on RCGroups.