This paper presents an advanced three-stage Doherty amplifier (DPA), using three 10W packaged GaN-HEMT devices from CREE. The amplifier incorporates three quarter-wave impedance transformers in the output combining circuit. The output matching networks consist of micro-strip line, lumped-parameter components and offset lines of the carrier and peaking cells that are used for proper load modulation. The transmission line in the input path of the carrier cell is inserted to adjust the delay among the carrier and peaking cells. The overall behavior of the three-stage DPA with the target of high drain efficiency at 6 and 8dB output power back-off is compared with a two-stage Doherty power amplifier and single ended power amplifier, while operating at 3.6 GHz by using Agilent's Advanced Design System simulation. The designed three-stage power amplifier exhibits a power-added efficiency (PAE) of 53.75% at full output power, and 50% at 8dB back-off. The power amplifier is capable of delivering up to 46dBm of output power.