With multiple Obi's set up with the same GV credentials, the last device to connect will be the one that rings. You can fork the incoming calls to the other Obi's from the first one on the Obitalk service.
The bad thing is the incoming caller id on the other Obi's will be the ID of the Obi that is forking the call.
Remove or disable the GV configuration from Obi #2 & Obi #3 so that they don't take priority after a reboot.
On Obi #1 (if GV is set on SP1)
Voice Service > SP1 Service > X_InboundCallRoute
default callroute is ph
edit it to {ph,pp(200xxxxxx|200xxxxxx)}
where 200xxxxxx is the number for the other Obi
There is a Home to Ofice Phone Extension Wizard
http://www.obihai.com/pbxExtension.htmlI don't know what type of caller id will show on the extension. It is set for 2 Obi's but it might could be used with more by modifying the InboundCallRoute DigitMap once you know what the modified digitmap looks like after the wizard does the modification.