The problem here is that PT3 can only connect to one instance of PostgreSQL at a time. What you need to do is backup your PT2 database from 8.2 and restore it in 8.3, then PT3, connecting to your 8.3 database will be able to read it.
First get your PostgreSQL 8.2 running: edit the postgresql.conf file for the 8.2 database and set the port to 5433. Save the file and you should be able to start up PostgreSQL 8.2 now.
There's a detailed walkthrough on how to backup and restore databases via pgadmin3 available here, just make sure you're connected to and backing up the database from PostgreSQL 8.2.
Once you've backed up the PT2 database, restore it into PostgreSQL 8.3 (same directions, just reconnect to the 8.3 database instead which is still on port 5432).
Once the database is restored PT3 will be able to find it in the Database --> Convert --> PokerTracker 2 Database window.