In Zenoss 2.5.2, I was able to create a custom default dashboard as listed here:

thread/7082;jsessionid=E6675672E6A3173CAF7898D659AB9F12.node0?decorator=print&displayFullThread=true

The Zenoss support team helped me with this in Zenoss 3.0:

First, go into the 'Users' section and select a user
that has the dashboard configured the way that you want it inherited. From there, append '/manage' to the end of the URL. This should bring up the ZMI
interface for that user's management. Select the 'Properties' tab in the upper right, and copy the contents of the 'dashboardState'.<br />

As the zenoss user, open the following file:

$ZENHOME/Products/ZenModel/UserSettings.py

Drop down to line# 563, you should be at a line that looks like this:

dashboardState = ''

You want to edit that line, pasting the dashboardState string value between the quotes that you copied out of the users's ZMI previously.

Save and close the file, and then restart the zenoss services. ( service zenoss stop | start )

Enjoy !