The only way you will learn how much bow weight you have lost or gained is to accurately measure your draw length and compare it with your draw length before you changed. The usual way to measure draw length is to have someone mark one of your arrows where it crosses the back of the bow while you are at full draw (the back of the bow is the side that is away from you as you draw). Measure the distance from the mark to the valley of the nock (the bottom of the slot in the nock). From your previous posts, I’m not sure you have ever done this, so you may not have the previous information, but it is a good thing to keep track of going forward. It is not a given that you're going to lose draw weight switching to 3 under, but it is possible.
As to how you adjust, that depends on how you aim. If you aim instinctively, you will initially hit lower because your brain has been programmed for the way you used to hold the arrow, and it will take a while to reprogram, but just keep shooting and it will eventually happen. If you aim by reference to the point of the arrow, you just have to establish new gaps, which is a much quicker process.