You need to be looking at power, not current. You will never be drawing full motor current, at full supply voltage. The motor is rated at 4.17V, 2.8A. Power is voltage times current, so the max motor power is 4.17 x 2.8 = 11.7W. your power supply is 36V, and you're measuring 0.5A, so the power is 36 * 0.5 = 18W. Close enough. You are running at, if not above, the max ratings for the motor. Two of these motors would double the power, to 36W. If your 6A motor is also rated 4.17V, then its power will be 25W. Your power supply is rated 36V, 8.8A, which is 316W. All three motors combined draw 18 + 18 + 25 = 61W, so your 8.8A supply should be more than adequate.

Regards,

Ray L.