You SHOULD install a 4 wire outlet and retrofit the dryer cord(easy). A 3 wire system(which is no longer legal to install for good reason) Has the neutral(white) connected to the plug and the ground is not used and CANNOT connect to the receptical(tho if metal box must connect to box.)
A three wire system used the neutral as a ground(or is it ground for a neutral). It is grandfathered in for previous installs, but should be upgraded if practical to a 4 wire system in which you have 2 hots a neutral and a ground.
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