This paper examines a FEL design for the production of three soft x-ray pulses from a single electron beam suitable for four-wave mixing experiments. Independent control of the wavelength, timing and angle of incidence of the three ultra-short, ultra-intense pulses with exquisite synchronization is critical. A process of selective amplification where a chirped electron beam and a tapered undulator are used to isolate the gain region to only a short fraction of the electron beam is explored in detail. Numerical particle simulations are used to demonstrate the essential features of this scheme in the context of the LCLS-II design study. Show Partial Abstract