Can't believe anyone could rate III worse than II.
Secret of the Ooze has no real plot, Sesame Street rejects as monsters, and basically ignores every single thing about the first movie to try to appeal to little kids instead. The violence is toned down from proper martial arts to hitting people with yo-yos. Even Donatello's voice is awful.
Whereas the third movie, sure it has lame Turtle costumes, but it does have a story, and the fight scenes are not as slapstick (although there is a bit of comedy fighting in all the Turtles movies) - Leonardo actually gets a sword fight in this one. The feudal Japan locations are a wonderful change to fake looking sewer sets. The villains are not comedy monsters. It's not a good movie by any means (I'd rate it maybe 5 or 6 out of ten) but it's simply less lame than Turtles II.
In addition, Secret of the Ooze is the worst Turtles movie by default as Casey Jones isn't in it.