It depends _a lot_ on which position your trefoil is in when you perform the move. Just like a crossing change or any other surgery-related change depends on where you perform it.
However, if you are thinking of a trefoil in the classical position with three crossings, then _yes_ , the result is an unknot (you can unknot it by three Reidemeister I moves).
Using this you can now see how the result could be different: since the Delta move is local, you can perform it on a factor in a connected sum. If you perform it on the second factor in the sum $$3_1\sharp \ U$$ where $U$ is an unknot in a position as described above, you can obtain $$3_1\sharp\ 3_1$$ that is a sum of two trefoils, as the result of a Delta move on one trefoil.