As long as the phpbb3 site is installed on the same webserver as the Joomla/CB site you can do that...
1) Install and setup Community Builder ( CB ) if you don't already have it
2) Install CBC from the CB Plugin Managment Screen
3) Click on Continue... then click on CB Connector in the plugins list...
4) Now adjust parameters for CBC as desired... setup the correct forum directory, plugin type ( phpbb3 ), etc.
5) Once all correct then click on the "Plugin Control Panel" link in the parameters column... this will open up another window with the control panel... from here you can Import, Export, Sync, etc...
Users with invalid Joomla usernames will not sync and I don't currently have an easy way to change it in the control panel... so those users would need to have their usernames changed in phpbb3 then import...
Greg