Here is a drawing of a homebuilt trailer hitch for a 3 point. I haven't actually tried to build it yet, but the kind fellow who sent me this drawing, "llamas", did and apparently it works just great. Pretty straightforward, but having the drawing does simplify things a little. You might want to check the dimensions against your tractor, but I would imagine there won't be any great differences for average size tractors.
Here is a post to the discussion group about a commercial version of the same idea. Don't know how much they cost, their website doesn't give that, but refers you to dealers. I note they have two different widths.
From: nward19us
Date: Thu Feb 28, 2002 6:10 am
Subject: 3pt hitch adaptor
In reference to the "home made hitch" posting on this club site, club members might like to know that
we manufacture and sell a patented hitch which turns
your 3pt hitch into a receiver hitch. It is designed to be used with your pickup truck inserts to tow
behind your tractor what you've been towing behind
your truck. Can alsobe used with sand/salt & seedspreaders,
gooseneck(optional) & regular trailers, weight racks, winches, etc. You can view it on our
website, "www.summitfab.com"