From a quick look at the differences in the code, it looks like Edit3 uses an overlay window to display the preview of the problem instead of switching to a new window. Personally, I find that difficult to use, but others may feel differently. There is also an option in Edit2 to append the problem to a homework set. I suspect that this was added to Edit2 after Edit3 was split off, and the option was never ported to Edit3.
PREP 2014 Question Authoring - Archived
Edit2 vs. Edit3
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