The database structure has not changed, so DT3 should seamlessly open DT2 databases (having said that, I can’t actually say that DT2 didn’t use a different structure in 2017, which would then have been automatically translated during later use of DT2). What’s the ending to the file name? How have you tried opening them (i.e. File/Open Database or other methods)? Is anything reported in the log?
The databases shouwl not be in a Dropbox folder or any cloud synced location. Remove them to the Databases folder in your Home directory or you could irreparably damage them.