You can't use z4root on Gingerbread. It is a Froyo root file. below is the link you will need. very easy takes just a couple of mins..
http://rootzwiki.com/showthread.php?3714-ROOT-Droid-3-root-instructions-(One-Click-added-for-Windows-Linux-OSX)&p=75866
http://rootzwiki.com/showthread.php?3714-ROOT-Droid-3-root-instructions-(One-Click-added-for-Windows-Linux-OSX)&p=75866