If you use PokerTracker 3 on two computers which are not always networked so you have separate databases on each computer but you want to keep your databases synchronized (so that you have the same information on both computers) then this is probably the simplest method.
You have two completely independent installations of PT3 on two separate computers.
You have some way of copying files between the computers. Often this will be over a network when you have your laptop at home for instance, but it can also be using a memory stick of some kind.
I will assume (for simplicity, and because it's probably the most common situation where this arises) that you have one computer permanently at home ("Desktop") and one portable computer that you use when you are away from home ("Laptop"), and that when you are at home you can connect them using a network.
You will configure PT3 to move the hand history (HH) files that it has imported to a 'Processed' folder, import the files from this folder into PT3 on the other computer, then manually move the HH files to another folder to store them for backup.
There will be a separate 'Processed' folder on each computer and a single backup/storage folder (although it obviously makes sense to keep a copy of this folder on both computers really).
You should configure PT3 on both computers in the same way.
Enable the Move Processed Files To option in the Auto Import Preferences. This will mean that when you have finished playing and stop Auto Import PT3 will move the hand history files that it has imported to the folder specified here.
Synchronizing Hand History Files
If you have played poker on your laptop while away from home and want to import the hands you played onto your desktop you can either share the 'Processed' folder so that it is available directly to your desktop via the network or move the files from the 'Processed' folder to a shared folder that you already have set up (or use a memory stick to move the files to the desktop).
Run PT3 on the desktop and use Manual Import to import directly from the 'Processed' folder on the laptop or from the shared folder that you moved the files to.
Manual Import does not move the files that have been imported, so when it finishes you will need to move the files yourself to another folder where you will keep all of the old Hand History files as a backup.
This will leave the 'Processed' folder (and the shared folder if you moved the files before importing them) empty so that next time you repeat this process you will only have new files to deal with.
Once you have imported hand history files to PT3 on both computers the files should all be in one place - the backup/storage folder. Again, I would recommend making a copy of this folder on both computers, but that can be done periodically if you wish.
If you have played on your desktop and want to import the hands to your laptop then repeat the process in the other direction.
Auto Import PT3 on laptop > Processed folder on laptop > [ shared folder on laptop > ] > Manual Import on desktop > move HH files to Backup/Storage folder.