I have used Traktor 2.5 several years too and recently installed T3 just to try it out, although mostly I still use 2.53.

Traktor 2.5 and 2 used the same "collection" database files I think, so 2.5 was able to use all your 2.0 information.

Traktor 3.0 uses a new collection database format (I think), so it builds a new database when you start it up. Normally it will copy the cuemarker and other track info from the 2.5 collection if that's what you tell it to do when you're setting it up initially.

That's not what I did when I upgraded to 3, I just searched for the MP3 files with T3 and imported them all. Worked great and very fast, T3 has all the beatgrids and cuemarkers and stripes without doing the (slow) analysis again.

If you just import MP3 files into Traktor 3 (like I did), it only has the information that is already in the MP3 file "tags". The tags can actually contain all the previous T2 information, such as beatmarkers, cuemarkers, and the "stripes" or graphic picture of the track waveform. That information is only placed in the MP3 files if you turn on that option (T2 menu: setup/browser/"Extended Tags & Stripe" button).

Also you need to make sure that just before you import tracks into T3, you have used T2 to write the stripes and tags to the MP3 files. Do this by selecting all tracks in the browser, then right click and and select "Write File Tag (ID3)". If you don't do this explicitly, some MP3 files will not contain the cuemarker/stripe info or they won't have the latest information which is kept in the T2 collection files. Make sure you unload the decks in T2 before writing tags also, or it won't write the info for those 2 tracks.