At the end of Guardians of the Galaxy, Groot sacrificed himself in order to save his teammates. Luckily, Rocket managed to save a twig, and Baby Groot was born. He'd become a teenager by the events of Avengers: Infinity War, and seemed only slightly older in Thor: Love and Thunder.
Vin Diesel has previously claimed that he'll be playing "Alpha Groot" in Guardians of the Galaxy Vol. 3, but writer and director James Gunn seems to have a different take on the matter.
Answering questions on Twitter, Gunn appeared to refute those claims when asked what the actor was talking about. "[No] idea. Maybe he just means the most awesome Groot ever? Vin was musing out loud - he hadn’t read Vol 3. What you saw at [Comic-Con] was Groot in Vol 3, and he’s not 'Alpha Groot.'"
Clearly, Diesel hasn't even recorded his lines for the threequel yet, so his comments about playing Alpha Groot may have been plucked out of thin air. Based on what we've heard from Comic-Con, Groot now very much resembles the version we saw in Guardians of the Galaxy, so it doesn't appear as if he's undergone too extreme a transformation.
This seemed to be confirmed by Gunn when a fan pointed out that they're hoping to see the adult Groot again. The filmmaker said, "Well, you just might be in luck..."
He'd later go on to share a few new details about the sentient tree's current status quo with his teammates. "In the scripts for crew and cast that don’t understand Groot it says 'I am Groot.' But I also have scripts available for those who understand Groot with the translations in them. [All] the Guardians understand him now. It takes a certain amount of time and bonding."
Gunn has previously confirmed that the trailer for the movie won't be released for a while because the visual effects are unfinished. However, we anticipate seeing it around the time Black Panther: Wakanda Forever is released this November (and failing that, it should drop to coincide with Avatar: The Way of Water the following month).
Guardians of the Galaxy Vol. 3 arrives in theaters on May 5, 2023.