What gauge ground wire should be used for a 220 V 3 HP motor in a well 400 ft deep?

No responsible person will answer this question without observing the situation first-hand. If you don't know how to calculate the answer yourself or look up the table in your local code books, then you shouldn't be installing a motor that size. 220V, 3HP motors are going to be multi-phasic, and that's going to require special connections.

Consult a local electrician.