In theory neither should mess the other up. I understand that HM sorted out the installation issue they used to have, but I haven't heard of any problems after installing PT3 first.
However, if you're not copying an existing PostgreSQL "data" folder across (but will be either creating new databases or using backup/restore) then you may prefer to install PostgreSQL yourself before running either. That way you can choose what PostgreSQL version to install, and set it up yourself. If you now have 64-bit Windows for instance, you may want to go for PostgreSQL 9.0 as that has a 64-bit version which will give you a bit of a performance improvement. (Do NOT use 9.1 yet as it is still pretty new and potentially still has problems which affect PT.)
Should you want to do this you can follow the install part of the uninstall/reinstall guide in the
documentation - just download 9.0 instead of 8.3 or 8.4. The process is basically the same.
If you're not confident to do that, the easiest way will be to run PT3 first and it will set it all up for you, then the PostgreSQL password will be 'dbpass' when you want to connect from another application.