Altostratus
MVPkristine_v The answer to you adding the new pool member in a disabled state is completely up to you and dependent on testing and policies. Typically before a server is added into rotation it is tested to ensure that it is already in a working state. In my previous experience the above was the case and we simply added the server to rotation and then performed load balancing testing during a set window where a possible outage or degraded service could occur. If you feel that placing the pool member is what is best for your environmen then I would go that route.
In regards to testing the server by itself in the pool, what you have does seem like the most logical option assuming that the other two nodes can be disabled for everything they are configured under on the F5. Disabling a pool member at the node level will remove it from rotation for every piece of configuration so make sure that that is the desired result. Once you have completed your testing ensure that old nodes are enabled so they can start accepting traffic. When going this route you should have the following steps.
1. Disable old nodes
2. Enable new pool member
3. Test with single node
4. Enable old nodes
5. Test with all pool members active and ensure that the testers have managed to use all pool members in rotation for each pool
On a side note, I would ensure that your load balancing method is set to one that uses the node level since you have a node that is in use across multiple pools so that you ensure traffic is evenly distributed across all your nodes. Previously I have used least connections (node) without issues but again the least connections part is up to you and your policies and what works best for the environment.