If the DVD has the space to hold the three movies then you can fit them on there. Combining DVD movies is not like adding tracks on a CD. DVDs have pretty strict rules about what goes where = authoring. You will need to reauthor the DVD, creating a new VIDEO_TS folder accommodating your three movies together so it will play on your television DVD player. This is not just putting all the files in one folder, but creating correct information files to go with the video. Even if you have a tool that will re-author for you it may not create menus, which makes navigating using your TV pretty cumbersome. Sizzle is a freeware tool that lets you create new menus, but it hasn't been upgraded in a while and is not the most intuitive. If you're happy with just the most basic of menus and working out problems with software then you can use it.
So, simply copying the three sets of files onto a single DVD will not work in your video player, though you can play the files on your computer using a program like VLC.
The other problem you may be encountering is blank media quality. If you are using the cheap DVDs sold in bulk in discount stores you may find your player won't be able to read them. I have put the same movie on cheap DVDs and high-quality ones. The player could not read the chap DVDs but had no problem with the identical files on a good quality DVD. RW media (be it DVD or CD) can often present even more of a problem in players than non-rewritable media, unless you player specifically states it can handle RW.
