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The T57 3in Gun Motor Carriage was the second attempt to fit a 3in gun on the chassis of the M3 light tank, following on from the T56. Both vehicles were built around the fuselage of the M3A3, the last production version of the M3 with a greatly enlarged superstructure. On both the T56 and the T57 the engine was moved from the rear to the centre of the vehicle and the 3in gun was mounted at the rear. The rear panel of the superstructure hinged back and down to provide a crew platform for the gunners. On the T56 the gun was protected by a boxy shield. The T56 underwent tests at the Aberdeen Proving Ground, where it was found to be underpowered.
The T57 was produced in an attempt to solve this problem. The gun shield was removed to save weight, and the 250hp Continental W-670 engine of the Light Tank M3 was replaced with a 340hp Wright Continental R-975 engine. The T57 went for tests at the Aberdeen Proving Ground but the vehicles performance was still unsatisfactory and both the T56 and T57 programmes were cancelled in February 1943.