This paper numerically studies the Mach number effect on cruise performance of 3D Co-Flow Jet (CFJ) wings at freestream Mach number of 0.15, 0.30, 0.35, 0.40, 0.46 and 0.50. The non-swept wing with three aspect ratios (AR) of 5, 10 and 20 based on CFJ-NACA-6421 airfoil are investigated. The numerical simulations employ the intensively validated in house FASIP CFD code, which utilizes a 3D RANS solver with Spalart-Allmaras (S-A) turbulence model, 3rd order WENO scheme for the inviscid fluxes, and 2nd order central differencing for the viscous terms. Similar to the previous 2D study, the 3D lift coefficient is increased with the Mach number due to compressibility effect. However, the overall aerodynamic efficiency and productivity are substantially dropped from 2D to 3D due to increased induced drag and the CFJ power coefficient. The penalty is further increased with the decreased aspect ratio. Compared with the baseline 3D wing with NACA-6421 airfoil at the same aspect ratio, the cruise lift coefficient is increased on the average by about 30% for AR of 5 and 60% for AR of 20. For the AR of 20, the pure aerodynamic lift to drag ratio CL/CD is also increased by about 30%. Considering the CFJ pumping power consumption, the equivalent aerodynamic efficiency (CL/CD)c is about the same as the baseline wing. The productivity efficiency (C2L/CD)c are increased by about 30% due to the increased lift coefficient. For AR of 5, no efficiency advantage is obtained even though the high cruise lift coefficient remains. The study indicates that the CFJ wing with a thick airfoil is advantageous for cruise at subsonic speed up to Mach number of 0.50, in particular for high aspect ratio wing.