Jamieson, Fausset and Brown on Galatians 3:5

5. He ... that ministereth--or "supplieth," God (2Co 9:10). He who supplied and supplies to you the Spirit still, to the present time. These miracles do not prove grace to be in the heart (Mr 9:38, 39). He speaks of these miracles as a matter of unquestioned notoriety among those addressed; an undesigned proof of their genuineness (compare 1Co 12:1-31).

worketh miracles among you--rather, "IN you," as Ga 2:8; Mt 14:2; Ep 2:2; Php 2:13; at your conversion and since [Alford].

doeth he it by the works of the law--that is, as a consequence resulting from (so the Greek) the works of the law (compare Ga 3:2). This cannot be because the law was then unknown to you when you received those gifts of the Spirit.