Are you sure you said 3000 users ? You mean concurrent users then...
Care with pgpool II. Not operable and write sync issues. Abandonned in our project. Look for related discussions in forum (there are, discussed with Penny Leach among others).
MySQL clusters have dimensional limitations and running constraints, usable but not for all tables if you need a sustainable and long uptime.
LB on NAS may not be usefull, depends on stress of file system on big file downloads.