Does the 3pt jump as in it's trying to maintain a certain height, this would be a leak in the lift cylinder.
Or, you can't set the position of the implement and when you try, it falls all the way down or raises all the up. If this is the case, then you are using the draft lever, the one on the inside of the quadrant OR you have a problem with your linkage. You could also check to see it the top link draft yoke has slop in it. That's the one with the big spring on the top of the axle housing.