Here are some steps to repeat a bug that I don't remember being in V3.

Open a 3DM file and select some curves.
Click the Export icon and notice that the 'File name:' box isn't filled in with the currently loaded file name.
From the list of files in the folder, choose the currently loaded file name.
Change the 'Save as type:' to DXF and click 'Save'.
A warning 'Confirm Save As' box appears stating "xxxxxxx.3dm already exists. Do you want to replace it?"
If you happen to click 'Yes', then your 3DM file is replaced with just the selected curves.


Martin.