DNS resolving OR port forwarding for web page request to related computer

I have software development background and little understanding setting up. Its a learning project and there are a lot of other things that we will be doing onwards. Let me try to explain what I want to achieve, I have 3 separate domains registered (please note its not sub domain) and pointing to my Static IP. On my main router I can forward port 80 to a computer (Windows 10 Pro) with IIS and website setup. It all works fine and I can load the web page. I have 2 more computers with Windows 10 Pro, IIS and some other websites are configured there as well, but we can't port forward to these computers as we know we can port forward one at a time. You will be helping me to setup some solution to overcome this problem, I have OPNSense firewall or anything else you can think of? I also have HP and Cisco switches to create separate networks (VLAN), you can also advise if adding some other hardware can make it simple? What I DO NOT want to use is, OpenWrt and reverse proxy, I do not want to use NGINX. I'm looking for some DNS resolving solution, not sure if we can use DNS service on firewall (OPNSense) or we need to setup a separate routine for resolving domains.