Ok, I dont know abt DIVX but I am telling you what I know.... here goes....
Regular MPG, AVI formats can be written to a DVD and the resulting video will be a DVD Video. This is not possible directly through Nero.
You need to buy a DVD Video plug in. and you need Nero vision Xpress3. Download the whole suite of NERO from
http://www.nero.com/eng/nero-up.php. Download ALL.
Then try running Nero Vision Xpress. You will get a message saying that you need a DVD-video plug in. It will give you a link to follow. Follow that link, pay by a credit card(any 9reputed one. Even issued in India will be fine) and then you will get an email with the code.
With instructions as to how to use that new serial number.
Then open Nero Vision Xpress3. Or Nero 6 click on "DVD-Video".
Then it is easy...... You can get a DVD playable on stand alone DVD players.
and abt the size, I suggest you use AutoGK to resize the whole movie.
Download Auto GK first and then run it. You will get an option to convert a movie into another size(not resolution but MB/GB).
Make each movie into abt 1 GB and then try ti burn a DVD-Video. Note that a DVD-Video of 4.7GB will actully contain less data than DVD-ROM of 4.7GB.
So, to fit 3 movies on a single DVD Video, you may need to use the trial and error method to get the final size of each movie.
Also, (obviously), when you decrease the size of the movies, the quality also decreases.....