Can you use 2.x on a 3.x database or it is 3.x only once you open it with 3.x?
The databases are basically the same and compatible. However, if you edit an item in version 2 which uses data of version 3 (especially custom metadata or reminders) then this data is lost.
When beta testing, it’s always best to use copies of data or test data. I personally also advocate running DEVONthink 3 on a separate macOS User account to keep things isolated. Your data, your choice - but I always err on the side of caution with peoples’ data.