for the 3 phase signal i think is best to use an arduino
i imagine the following program
a counter that goes from 0 to 359 and return to zero where at 0, 120 and 240 the pulses a,b,c starts respectively and they can have a duration that goes from 0 up to maybe 180 pulses each...
the frequency can be controlled by the timing between each count...
with the arduino yet can be done another button for gating it...
i guess will be necessary to gate it somehow to allow the cores to reset...