I have 3 Identical ESXi Host ...
Can you confirm that they are completely identical, including BIOS, settings, and CPU-Microcode?
Which version/build of vCenter, and ESXi are you using?
You will certainly be able to add the 3rd host to the cluster after disabling EVC. However, if something doesn't match, you may not be able to enable EVC for the cluster again, which may not be what you want.
What you may try is to create a new, temporary cluster with only the 3rd host. Then check whether EVC can be enabled with the required level. If this works, disable EVC again, and try to move the host to the original cluster.
André