If I understand what is being asked, you need to replicate what is working now three more times, so each DID needs:
- a separate inbound route with the DID specified exactly as it is delivered by your sip provider directed to ...
- a separate Set CallerID that prepends a different numeric prefix for each case
Then call flow from the Set CallerID continues as it is currently working. No additional extensions or outbound routes are necessary unless other circumstances dictate so.
This is FreePBX 101 basics and all answered in the wiki: