Summary form only given. Two high performance gyro-TWT amplifiers are being constructed at UCD. A third-harmonic gyro-TWT amplifier operating in the TE/sub 41/ mode will extend the proven concept that the electron beam current can be significantly higher in a stable harmonic gyro-TWT amplifier than at the fundamental due to the comparatively weaker harmonic interaction. Our previous second-harmonic gyro-TWT stably generated a record breaking output power of 210 kW. For stability, the circuits are sliced to suppress electromagnetic modes without the desired symmetry and the 100 kV, 100 A v/sub /spl perp///v/sub z/=1.35 electron beam from a magnetron injection gun is placed at the null of the strongest remaining gyro-BWO mode. The single-stage gyro-TWT is predicted to produce 2 MW with 20% efficiency, 30 dB saturated gain and 5% bandwidth.