PostgreSQL needs to be installed in order for PT3 to function properly.
If it exists, please run /Library/PostgreSQL/8.4/uninstall-postgresql, then please run the following three commands in Terminal (in Applications -> Utilities) - just paste each line and enter your mac password when prompted and hit enter (it won't show *s but it is working):
sudo /sbin/SystemStarter stop postgresql-8.4
sudo rm -rf "/Applications/PostgreSQL 8.4" /etc/postgres-reg.ini /Library/StartupItems/postgresql-8.4 /Library/PostgreSQL/8.4
sudo dscl . delete /users/postgres
Then reboot your machine and reinstall from a direct download available
here and it should install and then everything should work.