Hi Rich.
You only need two channels ,"North" and "East" and then transform these to UVW.
So If I generate 100% of north and 0% of east, the compass card will point North, I need 100% in the U winding and zero in the other two.
If I want to point at 120degrees around (South of East) then I put -50% in North and 86.6% in East.
(or something like that) Read the maths about page 6 of this:
http://www.ddc-web.com/documents/synhdbk.pdf See attached schematic that replaces , say, your yaw channel.
Its just a Scott T transformer, it works the same with rotating 3 phase power (used in motors) , as well as single phase with 3 amplitudes.
Here's a way of generating the two 400Hz waveforms (N and E) and combining them.
You can do the conversion with resistors and opamps, if you would prefer to use transformers with the same turns ratio.
Note that speaker transformers for use in PA systems, have multiple taps, so you should fins something close enough to 0.866 turns ratio, (even if this is not exact you can run a look up table for calibration)
So your software would be passed theta, and you output sin(theta) on North and cos(theta) on east.