Service Unavailable errors in SharePoint usually happen because the App
Pool Identity account has a password that has expired. When you setup
the app pool for the SSP did you use a Service Account or did you use
one of the pre-defined accounts like network Service?

I have three app pools, PORTAL, MySite and SSP.

They all map back to domain accounts:

MyDomain\PortalAppPool
MyDomain\MySiteAppPool
MyDomain\SSPAppPool

And they all have valid passwords.

can I tell if they have adequate access to the DB server by trying to
log into the database with their ID?

Does the same hold true for active directory.

Several points, just to try to cover all the possibilities...

1) Have the passwords to any of the accounts ever been changed? If
so, did you change them using Central Admin. If not, that is a problem
since these passwords need to be in the Configuration Database for
SharePoint.

2) Make sure that the accounts each are DBO of the content database
for their respective Web applications.

3) For AD, they should be a member of the Local administrator's
group on the WFE servers. Other than local admin they should also be a
member of the WSS_WPG group on the WFE.