The mirror M3 is a part of the in-vessel quasi-optical beam propagation system for the ITER electron cyclotron heating upper launcher (ECHUL). The millimeter waves are guided through fixed mirror sets (M1, M2, and M3) and the front steering mirror set (M4) to aim at the correct location in the plasma for suppression of the /1 NTMs. The design of the M3 shall guarantee the optimal propagation of the beams during their transmission taking into consideration: 1) thermal loads coming from the beams themselves and the plasma operation; 2) mechanical loads coming from the pressurized water circuits and the bolted joint; and 3) the port plug space restrictions in the front-end region. This article reports the main design features of the M3 and the fluid-dynamic analyses carried out to validate the capability to dissipate & x007E;30 kW of deposited power. Finally, the thermomechanical analyses are also reported to prove the design compliance to the structural design code for the ITER in-vessel components (SDC-IC) using a design by analysis (DBA) approach for the normal operation (NO) scenario.