You can get the latest version from the Download page.
The "Build" is the latest 'official' release, and will be listed first.
The "Beta" is the most recent version which includes all the latest bug fixes and features for testing. You will need to scroll down for the beta download.
(Both of these are 'full' releases and you can use either whether you are a registered or trial user.)
The first time you download PT3 you should get a "Full Installation", unless you already have PostgreSQL installed (e.g. if you use PostgreSQL databases in PT2). You NEED to have PostgreSQL installed once only - PostgreSQL is the database application that PT3 uses to store it's data.
After that, for every update, you should get the "Upgrade" version (without PostgreSQL) as you will already have it installed.
Once you have downloaded your selected version, make sure PT3 is NOT running and install the new version over the top - do NOT uninstall first.
If the installer reports that PT3 is running, use Task Manager / Processes tab to end any PokerTracker.exe processes.
Attention Microsoft Vista and Windows 7 users
Attn: Vista Users Installing PostgreSQL 8.3
Tutorial: Vista-How to Disable 'User Account Control' (UAC)