What you’re asking can be done. However, the quality would not be DVD, probably not even SVCD. It would be something like VHS. These are lengthy movies and require a fair amount of compression just to get one of them off a DVD9 and onto a DVD5.
To tackle the problem I would demux the originals into separate video and audio streams. If I wanted subtitles I would need to demux these too.
I would then use an authoring tool such as Vegas to concatenate the three movies and to re-render them in a format that would permit the new three-part movie to fit on one DVD. I would probably have to do some trial and error tests here, but almost certainly I would finish up with a smaller frame than the standard PAL or NTSC size used in DVDs. Moreover, I would have to pull the bitrate down to a low, low level. If you’re with me so far, you’ll realise that this is not a domain where quality reigns!
I would also need to decide whether playback would always take place on a computer, or whether a stand-alone DVD player would be used as well. The answer would influence the choice of CODEC (some are more efficient than others, but not all stand-alone players can ‘understand’ them).
The time and effort involved, to say nothing of the cost of the authoring tools and encoder(s) - which you clearly don’t possess, to arrive at such inevitably disappointing results militate against it.
I’d say forget it.