Sounds to me like your 'Selector Dial Pointer' is in the wrong position. I think the 3 position selector is behind the three point lift lever. It should have Depth, Height, and External/TCU. If you are set on External/TCU, the lift arms will lift 500 pounds when pushed all the way forward. The lift arms should go all the way down when the lever is held midway. This is the draft control for plowing.
You need to move the Selector to Height or Depth. In order to change the position of the Selector, the three point arms need to be ALL the way up.(higher than the normal fully up position) You can either lift them manually, or pull the lift lever all the way to the rear against the spring and the arms will be lifted a little higher than normal. While holding them in this position, you can change the selector lever.
I would highly suggest getting an owners manual.
You can also learn a lot right here, especially about common problem areas. Do a search for David Brown (or David, DB, etc.) and be sure to include the name "Elmer" in your search. Elmer is the expert on all models of David Brown tractors. If you search for his name, you will see all of his responses to everyone else's problems.