TOPIC:

We've migrated all of our vm's to a 3rd host and will be installing our new HA lizard enabled 2 hosts tomorrow. We would then need to add the 3rd host to that pool (temp) to migrate all of the virtual machines from the 3rd host over to the HA enabled hosts.

Will having the 3rd host in the pool have any effect on HA? Sorry if this sounds over cautious but want to be assured that we're doing it all correctly

That's how I have done it before. The 3rd host will have no effect on your other hosts or HA-Lizard.