If you only need low performance from the third gyro you could try integrating the output yourself. Allocate a regular AnalogChannel, poll it regularly, and add the values up.
Code:
pseudocode:
thisSample = gyroAnalog.getValue
thisTimeStamp = Timer.getTimestamp
thisAmountTurned = ((thisSample + lastSample)/2) * (thisTimestamp - lastTimestamp) * (Kgyro)
currentHeading += thisAmountTurned
lastSample = thisSample
lastTimeStamp = thisTimeStamp
lather, rinse, repeat
I doubt you will get performance anywhere near the FPGA, but it is pretty quick to try.