What you do is for SBC1 you create sip trunk to SP, with a Trunk Account and Trunk Group. Then you would create 3 Trunk Accounts to the SIP PBX, each with one of the IPs of the PBX. Then you would put all three of the Trunk Accounts in a Trunk Group going to PBX.
Trunk Group to PBX would look like this:
SBC2 would have the same configuration.
It would be up to the SP to route the inbound calls to SBC1 or SBC2, When the SBC receives a call it will send it to one IP on PBX, next call would go to the 2nd IP, and next to 3rd IP, then start back at 1.