Ditto what Dean said.. it's usually endemic of significant wear in the tranny.. It's not a bolt on part type fix.
If you want to bandaid it, attach a bunji cord or spring to the dash cowling, and have a hook end on it, so it holds pressure and keeps 3rd engaged.
I have an old beater ford 850 with loader that will occasionally jump out of 3rd, especially downhill if you let off the throttle. the spring method has been working for years. Again.. it's a bandaid.. not a fix..