I’m not aware of any change in the sync mechanism which would stop DTPO from syncing with DTTG3. The database format did not change between DTPO and DT3 either. DTPO can’t, however, sync via iCloud Kit of course (because DTPO simply doesn’t include iCloud Kit). I haven’t read of anybody actually doing what you are looking to do though - and I couldn’t find anything when I searched the forum.
Are you currently using DTTG2 or is this the first time you are installing DTTG?
May I ask why you have put off upgrading from DTPO to DT3?