The following contains spoilers for Solo: A Star Wars Story. If you don't want to know anything you don't want to read this until you've seen the film.
Anthony Daniels has been a part of every Star Wars to date and that streak won't end with Solo: A Star Wars Story. However, if you wondering how in the world the filmmakers are going to crowbar in a random appearance by C-3PO into the prequel story of a character that had never met them before Star Wars: A New Hope, the good news is, they won't. Instead, Anthony Daniels will appear in the film in an entirely different role. He still won't get to show his face, however, as he'll be appearing in a Wookiee suit. According to screenwriter Jonathan Kasdan...
No, but Anthony Daniels is in the movie as a different character. In the Papillon-type offshoot story, the Wookiee that helps them escape, Chewbacca's best friend, is played by Anthony Daniels -- and beckons him to join them as they escape on a different ship.
Solo: A Star Wars Story recently had its world premiere and so, while the vast majority of us have yet to see the newest Star Wars film, some have. Those people might have kept an eye out for everybody's favorite golden droid, as it had recently been reported that Anthony Daniels would be appearing in the film, but when C-3PO was nowhere to be found, Uproxx asked screenwriter Jonathan Kasdan about Daniels' appearance.
This works out well, because it keeps Anthony Daniels' streak of appearances in Star Wars movies intact, making him the franchise ironman, but doesn't require the movie to tack on a C-3PO appearance that honestly wouldn't make a lot of sense. It seems pretty clear in Star Wars: A New Hope that Han Solo has never met C-3PO or R2-D2 before, so having them have a run-in during Solo: A Star Wars Story would seem odd. Even throwing them into the corner of a scene where they don't actually interact with anybody, as was done in Rogue One: A Star Wars Story, would be awkward. At least in Rogue One, there was something of an understanding why the droids would be where they were without having to actually explain anything. That would be trickier to pull off in Solo since we're dealing with a story that exists, at least mostly, outside of the general "Imperial vs. Rebel" struggle of the other movies.
Keep your eyes open for the additional Wookiee's that we'll see in Solo: A Star Wars Story, because one of them is actually Anthony Daniels. Solo arrives in theaters May 25.