You don't have to permanently delete them to keep them from the DVD. If you make a cut in the video where you want your DVD to begin and end, then you can just delete the parts you don't want from the timeline. When you burn your DVD only the video you want will be on it, but all three will still be a part of the original file. If you need to have just that one video in a media file on your computer, simply open the Magix video project you used to burn the DVD, and click File>Export Movie, and you can make a completely seperate media file with only the one video you want to keep. 

Hope this helps