Yeah, I reused the same functionality that discobot uses for skipping emails. 
https://github.com/discourse/discourse/commit/31e2385316422a9d91ec118e73504143f3e4542e
And this is what I was working on:
https://github.com/discourse/discourse/commit/5210e3e744995ba94ba69fdf9a64adcb571be58d
Up until now if ONE replied to TWO and THREE, it would have been rejected by Discourse because the reply key didn’t belong to ONE. It belongs to THREE. But since Discourse knows that ONE was CC’d in step 3, it will now allow the incoming email from ONE.