when you dropped the third plow, did you drop the center plow? most times this is what folks do.
most times the rear plow has a longer landslide or trail furrow wheel that helps keep the plow running true or straight.
if you dropped the center plow it may have a shorter landslide, so when you add the 3pt hitch to the plow, it might not run straight without the longer land slide.
just a thought.