Patrick Troughton agreed to return, as did William Hartnell, though producer Barry Letts, after he called and got Hartnell's agreement to appear in the story, was soon called by Hartnell's family and told that the actor was in fact gravely ill, and that full participation wouldn't be possible. It was decided to pre-film Hartnell's scenes at his home, allowing him to appear and interact with his successors. It was Hartnell's final appearance in the series, as he passed away two years later.
While The Three Doctors as a whole is not top shelf Doctor Who, it does feature many great moments, most of which involve the bickering banter between Troughton and Pertwee, who enjoyed good-naturedly ribbing each other in real life. Check out our first scene, when the Third Doctor and his companion Jo meet the Second:
I love that part with "I Am the Walrus."
Finally, we've got a scene with all three Doctors together: