I got to admit I have never even heard anyone use the term "triple redundant endpoints"
My first reaction was ->
I'd imagine it might be possible to configure more than 1 peer IP address for the L2L VPN connection. I have never really checked how it works
On to the topic,
Do you mean you have the local network 172.16.2.0/24 network the support company has to reach? If thats the case then the overlapping networks aint a problem. You can NAT the whole network to some other private address range /24 network before the traffic enters the new L2L VPN tunnel.
Also to even help you abit I would need to know:
What device would you be using for the L2L VPN?
What software is the device running?
Have you agreed on the Phase1 and Phase2 parameters (except for the network that will be visible to the support company)