If a person receives a cash gift from an NRA before he himself becomes a resident, does he have to report that gift on the 3520 if he becomes a resident later that calendar year? I would think that this is a reportable event but not taxable?

Form 3520 is used to report transfers of money from OUTSIDE of the United States. I believe the threshold for filing Form 3520 is $10,000, but it may be higher.

If you were a non-resident alien at the time you received the gift, it need NOT be reported to the IRS, even if you file as a duial-status alien.