What Happened to Sandman in Spider-Man 3?
Sandman, aka Flint Marko, showed up as one of three villains in Spider-Man 3, the last of the Tobey Maguire trilogy. In that continuity, he's revealed to be the petty criminal who killed Uncle Ben during a carjacking gone wrong. On the run from police after escaping, he falls into a particle accelerator that fuses his body with sand, giving him the ability to reform his body as literal sand. Peter goes after him after discovering his link to Uncle Ben's death, and although he's nearly killed, he survives and allies with Venom to take down Spider-Man.
After the final battle, Marko apologises to Peter and explains that Uncle Ben's death was an accident; Marko had become a robber to pay for his young daughter's surgery, and he never meant to hurt anyone. Peter forgives him and allows him to escape.
Why is the Sandman in Spider-Man: No Way Home?
Sandman is only seen briefly in the No Way Home trailers, most visibly as a gigantic, sentient cloud of sand with a face. It's unclear how this fits into his story or why he's back as a villain after his exit in Spider-Man 3.