TSC and the like, used to sell a drawbar bracket for the older 8N Fords, because the 8N's didn't come from the factory with a drawbar. Instead they had the 3pt drawbar like you have, with stabilizer bars that fixed the 3pt arms in place. I don't know if this bracket would match, but it may be "re-workable" for your Yanmar.
You might find it easier to add the stabilizer bars, which are flat steel about 1/4" x 1" that are pinned at one end over the pin at the end of the 3pt drawbar, and then at the other end into the tractor-end of the top arm. This forms a triangle that won't let the three point arms move. Now that worked because the hyd. system allowed it to work. Not sure if the Yanmar would like its 3pt arms tied down that way. It might try to break something. There was a picture of this arrangement posted recently on TBN and I think it was in the discussion of turnbuckles (chains) vs. flat adjustable bars for anti-sway of 3pt equipment. I will look.
On the 8N, when the arms were tied down, it allowed a one-way ram (maybe a two way, but don't think so) to be plumbed into the system so the ram would work when the 3pt quadrant lever was raised.
