Here's a really obscure one from Gottlieb: Amazon Hunt, Amazon Hunt II, and Amazon Hunt III. Look it up.
Road Show is arguably part of the Lawlor Disaster Series. With the earthquake.
Stern Electronics Dracula, BSD, and Monster Bash could be a trilogy.
X-men, Tron, and Rush could be a Borg trilogy, sort of. Or something.
Rollergames, WWF, and WWE are all theatrical-versions-of-sports games. Or Tag-Team Pinball, WWF, and WWE.
Grand Prix, Indy 500, and Mario Andretti could be a Formula 1 trilogy. There are other racing trilogies that could be done as well.
Various options for basketball trilogies.
BSD, Freddy's Nightmare on Elm Street, and JCH could be a "serious horror" trilogy.
Speaking of horror: Barracora, Space Invaders, and Alien could be an HR Giger trilogy.
Rocky, Ali, and Champion Pub. Or Title Fight.
James Bond (Gottlieb), Goldeneye, and James Bond (Stern) (upcoming)
Iron Man, Avengers, and AIQ for the three games that feature Iron Man and Black Widow (she's in AIQ, right?)
Godzilla (sega), Godzilla (stern), and Ultraman for a Kaiju trilogy.
Medieval Madness, X-men Premium/LE, and Hobbit for a pop up target trilogy.
I'm going to sleep now. There are probably more.