First, if you think you might want to go back to DU2 for something, you'll still need that address. Open another window/tab in your browser, click to your current home page (DU2), and then make a bookmark for DU2 using the Bookmark function in your browser.
Next, come back to this window. Hit the "home" tab
in DU3 to take you to the front page for DU3--or Latest Threads, or wherever you like to start. Copy the URL from that page, then open the Preferences for your browser and paste the DU3 URL as your new start page.
Questions like this should be asked in the Help & Meta-discussion forum, which can be found here:
http://www.democraticunderground.com/?com=forum&id=1240
If you need more help and can't reply to me here because another host has locked this thread, please feel free to PM me or post in Help & Meta.