The Mark III has been improved both by the installation of a new thermionic microwave gun and added capabilities. The new electron gun yields a reduced emittance and allows operation at a higher repetition rate and an increased electron macropulse length. We have added the capability to tailor the input microwave pulse by adding a ramp or a step onto the macropulse. Phased-locked operation has been used to produce tunable optical radiation with filtered spectral widths of 200 MHz. Both phase-locking and hole-coupling have been used to increase the outcoupling efficiency. The RF system of the Mark III is now phase-locked to the RF system of the adjacent storage ring driver for the short-wavelength FEL sources of the laboratory, making possible two-color UV-IR pump-probe experiments.