I needed to rotate a 3gp video. I tried several free solutions: ffmpeg, WinFF, Avidemux (v. 2.5). None worked. They all complained about the input file.
Finally I found Aimersoft Video Converter (v. 4.1.2). Problem solved! Then I converted and rotated some more 3gp-s and mov-s. All easy and without a single issue. Great program!