Since you've just started trad shooting, nothing is really ingrained yet, so it is a good time to try different things to see what works best. It should be easy enough to lower your POI by raising your anchor point, if that's what you want to do. However, 18m is a pretty short point-on distance. Mine, for example, is about 45 yards. With a 45 yard point on distance, one hits at 20 yards either by gapping at a point below the POI, or by instinctively holding low. Either way can be accurate, with practice, although perhaps not quite as accurate as holding on the mark. But then if you do something to change your point on distance to 18m, you're going to have to hold over at 25m, so you can take your choice.
Assuming you are consistently hitting 3" left (assume you are a RH shooter?), and not committing one of the many form errors that results in a left miss, then there are several things you could try to move your POI to the right 3".
Be sure your dominant eye is over the arrow, and not slightly to the left of the arrow. If the arrow seems to fly to the target at an angle rather than straight, then your eye is not over the arrow.
Try both weaker and stiffer spined shafts. The usual advice would be to try a weaker shaft, but sometimes the opposite works better when dealing with fletched arrows.
Try a softer strike plate. If you are using a leather or Velcro strike plate, try one made from a rug rest, such as a Martin Rug Rest.