The question is: What was the third Doctor Who story to have a dinosaur?
Before the answer, let's look at the other dinosaur appearances.
Invasion of the DinosaursHere Barry Letts and Terrance Dicks talk about what they were promised and what they eventually got.
Dinosaurs on a SpaceshipAnd "The Making of" video from the BBC
Answer : Mark of the Rani.
Even in 1985 the dinosaurs still had a way to go. But one thing you can say about the producers of Doctor Who is They are always willing to take another chance.
Guess third times a charm...