Greensomes is not formally recognised in the Rules of Golf so the R&A state that the Committee should specify the competition rules and conditions.
However unless the organising committee stipulate otherwise I'd guess it's the same as for Foursomes, covered by Rule 29:
Match Play - loss of hole.
Stroke Play - strokes played out of order are cancelled, two stroke penalty and stroke must be replayed in correct order. If the error isn't corrected before teeing off on the next hole - DQ.