"I originally wrote Lambshank, the character that I played, for Stan Lee because I wrote the movie before Stan passed on," Gunn explains in the movie's director's commentary (via The Direct), referring to one of the High Evolutionary (Chukwudi Iwuji)'s genetic experiments that the Guardians encounter on Counter-Earth in a bid to save Rocket (Bradley Cooper).
Gunn's commentary continues: "I knew he was also getting much older and it'd be harder to bring him to Atlanta to shoot, so I did a character that I could just animate and then have him do the voice and have a little Stan Lee-like face on that character. But unfortunately, Stan passed, who I really enjoyed working with and directing so many times through these movies."
The late Marvel Comics boss had a cameo in every MCU project from 2008's Iron Man until 2019's Avengers: Endgame. Lee passed away in November 2018 at the age of 95, but the first draft of Guardians 3 was finished back in June 2018 so Gunn would have seen no reason not to include a cameo for him. The movie was released back in May and was a hit at the box office, grossing $840 million.
The latest project from Marvel is Secret Invasion, which airs weekly on Disney Plus. While we wait for the latest installment, make sure you're up to date with the MCU with our guides to Marvel Phase 5 and Marvel Phase 6.