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I have a different take on your options. Since the Master bath is on the 3rd floor and not in the same space as the zone you want to connect it to you will have no control over the heat in the Master unless you install a thermostat for the Master. This leaves you with 2 options. Either install 2 zone valves, 1 for the original space and 1 for the Master or add a 3rd pump which sounds like a relatively easy option since you apparently have left provisions for an additional zone. I prefer pumps over zone valves.
Let me know if you have questions about control wiring/config. Or do you have any more questions?