Windows Movie Maker is the software you'll want to use for this. In Movie Maker, just click import video nd import the video that has the 3 second section that you want to change into pictures. Once you import it, drag it to the timeline. From there, trim the video down to the 3 second section. You can trim by just clicking the edge or the video clip and dragging it to trim. From there, you'll want to use a button that looks like a camera that is just below the preview screen for the video. By clicking that, it will take a picture of whatever is on the preview screen at that moment. From there, you just go along with the video, taking snapshots however much you want to, and in no time you will have that 3 second clip in a bunch of pictures. I hope that helped!
