This DVD style looks very promising for some movie backups.
I do not know if transcoding compression from 4.5GB to only 1.5GB still have excellent viewing, you want to ask that in the specific compression forum. Most people aim to transcode no lower than 80% although I have seen good results with 60%. I understand that when you say it plays on stanalone DVD player, this is plain-vanila player, with no special formats like MPEG4, etc. What program you use on PC and if you need special decoders. Are all the 3 movies full screen resolution? Is each movie about 90 minutes and longer?
Can you tell us more on this DVD? When you browse it with Window Explorer, does the file system look like normal DVD, with VIDEO_TS directory, *.IFO and *.VOB files?
Maybe someone can understand how it was done if they know the exact title. Was this DVD bought from some vendor or authored and burned by some individual?