First thing to worry about is how large is the actual movie download, and what is your data plan limit?
My experience with 3G was that it was never up to the download speeds I got over my broadband internet connection at home (LTE is a different story however).
As to how long it would take, no way to tell you. Just like any internet connection, realized 3G speeds vary widely depending on which network you are on, and where you actually are on that network.
It would depend on your carrier whether you can download such a large file over their data network. It would probably take up a significant portion of the monthly cap most carriers have in place for most plans, or even exceed it, so I would strongly recommend you review your cell plan to determine any such limits before you attempt it.
It may be more practical to try and sort out the problems with your WiFi connection.