The MCU connections in the upcoming third entry in the Rocky spin-off series, Creed III, has been heavily commented on, but they're a lot deeper than they seem. Some Creed III connections to the Marvel universe are obvious, with stars Michael B. Jordan and Jonathan Majors playing significant roles in the MCU. However, Creed III's ties to the MCU are more layered than just these two actors, with the film tying together Marvel heroes, villains, and more.
Creed III continues the story of Adonis Creed, the son of former boxing champion Apollo Creed, who died in Rocky IV. This time, Creed III will follow Adonis as he takes on an all-new foe from his past, with Jonathan Majors' mysterious character reappearing in his life and wanting to challenge Adonis to a fight. On top of that, Creed III won't feature Sylvester Stallone's Rocky Balboa, meaning that Adonis will take on his former friend without the help of his long-time boxing trainer. Creed III will surely be a significant film on its own terms, but the project also happens to bring together loads of performers from the MCU.
Creed III features six actors who have previously been in the MCU. Michael B. Jordan is Adonis Creed, but also plays the MCU's Killmonger, the main villain of Black Panther but also cameos in Black Panther: Wakanda Forever. Jonathan Majors is the villain of Creed III and is also one of the MCU's most prominent villains, playing Kang the Conqueror and his variants across multiple MCU projects. Tessa Thompson has a starring role in Creed III, having been in all three Creeds, but she also plays Valkyrie (aka King Brunnhilde) in the Thor movies and Avengers: Endgame.
Creed III also includes other MCU actors in supporting roles. Florian Munteanu is Viktor Drago in Creed II and will reappear in Creed III, with the actor also the minor villain Razor Fist in Shang-Chi and the Legend of the Ten Rings. Selenis Levya is in Creed III, and is the physics teacher Monica Warren in Spider-Man: Homecoming. Finally, Wood Harris appears in Creed III and the MCU, playing Officer Gale in Ant-Man and Ant-Man and the Wasp: Quantumania.
Creed III features Kang, Killmonger, and various other MCU actors, but it's probably not a conscious choice. It's more likely that, at this stage, there aren't that many actors who haven't appeared in the MCU before. The MCU has been ongoing since 2008, and with the franchise including 30 movies, multiple TV shows, and various specials, there aren't too many actors the series hasn't consumed. The MCU requires many actors across countless projects. Almost every new major movie inevitably features a handful of Marvel alumni, not just Creed III.