We also have three domains, and on our RTP segment configuration page, you can select the domain(s) from the drop-down in the upper right.
Furthermore, in the rule logic of that same segment configuration page, you can add an "include pages" rule that matches only URLs containing specific domain(s) (e.g. *ford.com*).
Finally, in the campaign setup, you can set the campaign to fire (after matching the segment above) only on URLs containing the domain of your choice.
So I'm confident there is some combination of how you choose your domain(s) in each of the 3 steps above that will result in the firing behavior you seek.