A lefty’s drag line that veers off to the 3B side indicates that he’s exerting force in that direction - probably in response to exerting force in the opposite direction somewhere else in his delivery. (Think physics: for every force there is an equal and opposite force.)
If the drag line is only slightly off-center, then it might not be an issue. Just move him on the rubber so that the drag line ends on the centerline between the rubber and home plate.
Otherwise, I’d look for some other movement he’s making that causes him to push toward 3B with the back leg, analyze it, and make any appropriate corrections. However, if he’s not getting it going and building up a good amount of momentum, you might want to focus on that and see what happens with the drag line. Remember that the drag line is a result - not a cause - so don’t try to change the drag line directly. Instead, change the rest of the mechanics and, hopefully, the drag line will take care of itself.