Long story short: Speed 3 is an amateur short film video about a bomb on a roller coaster. It's cut together faster than any MTV video ever made. Read the long story made long, from Roger Ebert himself:
No favorable review I've ever written has inspired more disbelief than my three stars for "Speed 2." Even its star, Sandra Bullock, started mentioning in interviews her disgust with herself for agreeing to star in it. It's frequently cited as an example of what a lousy critic I am. All the same, I'm grateful to movies that show me what I haven't seen before, and "Speed 2" had a cruise ship plowing right up the main street of a Caribbean village. (Note well: Siskel also gave it thumbs up.)
In the spring of 1999, at the University of Colorado's Conference on World Affairs, I argued that movies like this weren't as easy to make as it might appear. In a moment of madness I announced in self defense a contest for a new "Speed 3." The rules were: It should be on VHS, no longer than five minutes long, and must involve something that cannot stop moving.
Read the rest of the story here on Ebert's blog. For the record, he's right about the finale in which the ship crashes at the end of the movie. That scene was so awesome it knocked my socks off. The rest of the movie, however... chainsaws on a cruise ship. Not one. Plural. Seriously.