Sorry this took me so long to respond to; I had to play around a bit to find a solution.
I couldn't find a way to make AviSynth/Vdub work, so I downloaded a program called "SUPER ©." I've known about it before, but this was the first time I used it. Good news is that it works; bad news is that's it's sort of unintuitive (only sort of, though).
This is the install file for the program. Unless it comes with a ReadMe file (I didn't check), the closest thing I could find to a guide for it is their
FAQ. You may not need it, though.
At any rate, drag and drop the .3gp file into the program and set the output settings at the top of the program's GUI. Output container = AVI; Output codec = huffyYUV; Audio codec = (some sort of) PCM.
Change the other settings as you prefer.
That'll get you a lossless file that you can edit or whatever. If you aren't looking to edit, you can use something like DivX for the video codec and MP3 for the audio codec. It's lossy compression, but it's better for simple storage.
... yea ...