One option is to set the 3 devices on their own subnet and configure the web server to respond only to those addresses. If you need external access to the web server device from outside for monitoring, admin, etc. configure your router to allow port forwarding to that specific internal address, specify the data port, can be different than :80, and there you go.
Retired IT professional, programmer and "beardie weirdie".
RPi interests: Artificial neural networks using clustered devices.
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