ECV Health Monitor issue

The Lack of a Host header is certainly a problem. If there are multiple web sites hosted on the same servers accessible via different hostnames, then your monitor might be monitoring the default sites on those servers rather than the one you think you are monitoring.

If all three servers require different hostnames, I don't see how you could construct a single HTTP monitor to handle all three of them.

If you are putting your web servers behind a load balancer, you should configure them all to serve the same domain name.