Don't install PU3, it has issues.

Jay is right, if you don't need to extra security, go pack to PU1.  If you don't have that option then its very difficult to have more than one password Group Policy at the Domain Level in AD.  I have heard you can do this with FGPP - "Fine-Grained Password Policies" and apply them to specific users and groups however I have no pratical knowledge of this as I've never actually tried it.  Its a theoretical solution but you could research though I doubt Sage or any Business Partners are going to be able to assist.  At the end of the day, PU3 is all about hardening security and if the inconvenience of that security is too great, consider rolling back until Sage are able to build more flexibility into the solution.

With the release of Sage 300 2024, one can again set passwords to not expire however I believe the expiry period will remain linked to the Local Security Policy of the SQL server.

Also have a look at this post.