The Point
The most probable candidate I suspect- though not a full match to something barely remembered.
It has a medieval theme. A character with 3 faces (most of the others are triangular). It aired in the 1980s.
The Point (1971)
The Son of the White Mare
The 1981 Hungarian cartoon The Son of the White Mare has three brothers.
And this scene could be remembered as "triangularly stitched together" - though obviously most kids in 1981 wouldn't have seen this movie.
Gandahar
I look forward to an actual answer with a triangular protagonist.
However I'll propose that perhaps some memories are being mashed up. I hope the other users of this site will forgive an answer attempting to be helpful with multiple suggestions and pictures.
In addition to below suggestions other notable medieval fantasy may include "the Black Cauldron" or the non animated Krull or Dark Crystal.
The movie Gandahar (or Lightyears) has a multi-faced mutant. It also has some bird imagery. The end of the movie is the hero jumping into a brain but there's enough things in the movie that could be remembered as a cauldron.
He-Man
Alternatively episode of He-Man and the Masters of the Universe "Betrayal of Stratos" featuring his flying friend Stratos has eagle motifs and a demon in a cauldron. And since this was a series that raises the potential for more modified memories from multiple episodes.
Edit: Additional speculation noting that Man-E-Faces in Masters of the Universe can switch between three faces. Again I realize this isn't a great match to the original question - but no alternatives have been proposed.