This is a list of all the franchises I've watched that have three or more movies, of which I've seen at least two. I decided to not include any series with only two movies because I felt it would add too many. For simplicity, I'm treating the MCU, DCEU, and X-Men movies as singular franchises, and not separating by the series within them.
Many of these are incomplete. Here are the franchises I've fully watched:
Harry Potter, Back to the Future, DCEU, Toy Story, The Dark Knight trilogy, Despicable Me/Minions, Star Wars (OT, prequels, & sequels), Batman, Mission: Impossible, Lord of the Rings/The Hobbit, Ocean's, Unbreakable trilogy, Spider-Man (Raimi & Webb), Dollars trilogy, Dirty Harry, Mad Max, the Godfather trilogy, Three Flavours Cornetto trilogy