Back to the Future III
And you lost me.
BTTF3 wasn’t bad by any stretch. And it had a great arc for Doc Brown’s character.
Star Trek IV: The Voyage Home
Star Trek IV: The Voyage Home is the fourth film in a hexalogy.
But it’s often considered the third film in a trilogy-within-a-series, beginning with Wrath of Khan. It’s even sold this way in some box sets.
I own a box set sold this way, and the special features discuss people referring to it as a trilogy.