The sheet beam extended interaction klystron is an important kind of millimeter-wave and sub-millimeter-wave vacuum electron device, which has extensive applications such as in high resolution radar, imaging system, satellite communication and precision guided missiles. Compared with conventional pencil beam klystron, the sheet beam extended interaction klystron, in which a thin rectangular sheet beam is used, can generate higher power by obtaining higher current and reducing space-charge-effect of electron beam. Kinematical theory and space charge wave theory are extensively used to analyze the bunching process of electrons. Kinematical theory is precise when electron beam is especially small because the influence of space charge effect is ignored, while space charge wave theory is accurate when the modulation of electron beam is small since it is based on the premise of small amplitude. Electron flow oscillatory theory is a compromise between kinematical theory and space charge wave theory, which adapts to the bigger modulation of electron beam than space charge wave theory, while it is inaccurate in the case of big bunching parameter. Based on electron flow oscillatory theory under the small signal condition, the influence of electron beam on standing wave electric field of 2π mode in a three-gap cavity is analyzed, and the expressions of beam loading conductance and beam loading susceptance in a three-gap cavity are obtained. The influences of plasma frequency, transit angle of single gap and transit angle of drift on the interaction of beam and wave in a three-gap cavity are discussed. The results show that space-charge-effect of beam is unbeneficial to the interaction between beam and wave, otherwise beam loading conductance and beam loading susceptance fluctuate with the increasing of transit angle of single gap and transit angle of drift. A W-band sheet beam extended interaction klystron is designed by theoretical analysis and 3D PIC software. The output power of 5773 W at 94.47 GHz is obtained with an efficiency of 8.46%, a gain of 37.6 dB and a 3 dB bandwith of about 140 MHz, when beam voltage is 19.5 kV, current is 3.5 A and focus magnetic field is 0.85 T. This research is important for the engineering of the W-band sheet beam extended interaction klystron amplifier.