In a word, don't use Share. Buggy as can be.
If you want to create 3 1hr segments, Set the in / out markers in the appropriate location for a segement then export that segment as a self-contained file with "include all markers" selected.
Once you have the 3 files, use Compressor to generate your m2v and ac3 files and import them to DVDSP to build your 3 discs.
This process gives you complete control over the export, compression and authoring of the discs.