My 3520 is like his 3320 (stabilizers won't allow the 3ph arms to come together) so how about this and it won't cost any money.
Run a loose chain attaching it to each end of the arms on the 3ph. It needs to be long enough to go under the tongue. Back the tractor up to the spreader hitch allowing the chain to go under the tongue. Using the 3ph raise the spreader tongue to the height of the drawbar. Back the tractor up and set the pin. The chain should slide down the length of the tongue.
Is that clear as mud?
Run a loose chain attaching it to each end of the arms on the 3ph. It needs to be long enough to go under the tongue. Back the tractor up to the spreader hitch allowing the chain to go under the tongue. Using the 3ph raise the spreader tongue to the height of the drawbar. Back the tractor up and set the pin. The chain should slide down the length of the tongue.
Is that clear as mud?