Let me try to see if I understand you correctly. Which one is it:
you want to replace IPsec with OpenVPN for a network topology consisting of 3 sites, each of which will be communicating directly with the other two, or
you want to keep the current IPsec VPN setup, and just add a OpenVPN remote-access functionality (so that people can connect from e.g. home) to the main site, but you also want remote workers to be able to connect to LAN IPs at all three sites.
In the first case, keep in mind that you can't have a fully-routed topology and use both IPsec and OpenVPN at the same time.
In the second scenario, you'd need to add IPsec P-2 entries for the OpenVPN roadwarrior subnet at both site-1 and site-3, and push appropriate routes to your OpenVPN clients (assuming you're not redirecting all their traffic to go via the VPN).