All of them in any amount and yes, your specific 3 seconds should be labeled as being AI generated in your video (or description) and your entire video should be labeled as made using AI by youtube.

A 30 minute speech by a president where I use AI to change 3 seconds in order to make that person say something they never did should also get the label. The label shouldn't be about how much AI was used, but that it was used at all.