3mm (0.1182") would be very close to 11 gauge (0.1196"). Although I never measured the thickness of an early Ford frame rail, it is said that Henry used 11 gauge for his production rails. 0.118" is also the N.H.R.A. minimum thickness for mild steel rollbars and rollcages.
75 x 50 x 3 should work, or if you need a little extra beef, 100 x 50 x 3.