First open /Applications -> iSync.app -> (Control-click and choose Show Package Contents) -> Contents -> Plugins -> ApplePhoneConduit.syncdevice -> Contents -> Plugins -> PhoneModelsSync.phoneplugin -> Contents -> Resources -> MetaClasses.plist and find the entry for the RAZR V3. Duplicate this section and modify all instances of V3 to read V3i (except in the icon name).
Now plug your phone into the USB cable, open iSync and select Add Device; you should see the RAZR V3i pop-up. Double-click to add, and you are sorted.
You may receive the message "service cannot be started," especially if you've been playing with Bluetooth prior to testing the USB cable. If you restart the phone, this will go away.

