How can I cut a few seconds at the beginning of a large video file? I have a large (~3GB) video file and I would like to trim just a few seconds at the beginning (~3s).

What would be the fastest way to do this without having to process all the video file?
Try this: 


Open the file in the Quicktime player -- it'll have to be Quicktime Pro.
Select the frames that you want to remove with the input and output markers.
Hit 'Delete.'
Then save as a 'self-contained' movie.