All could accept "the new normal" is going to feel weird until enough time has passed and becomes the "old normal" and agree to take it one thing at a time.
Could also ask him what his thoughts and comfort level are with affectionate displays and if he's prepared to grieve "the loss of exclusive." You will go through that too -- maybe to a different degree. Even when all choose to Open? Open is NOT CLOSED. Maybe this thread could help.
Maybe he's totally fine with it. And you are pre-worrying for nothing. Or if it is awkward for him, ask him how he wants your support while he works through that?
Because it is reasonable to expect affection toward your sweetie. Even when the sweetie is not him.
Again -- could go slow. You could assume responsibility for your conduct, and assume your responsibility for providing your partners with APPROPRIATE support/nurture in polyshipping.
Could guard against wanting to control their behavior or control how they feel or wanting to do their responsibilities for them. Let them have their own space for that. Everyone holds their own emotional baggage. Everyone holds up their end of the responsibility sticks.
It doesn't have to be a bigger deal than it is. Just play ball responsibly and enjoy the unfolding of your new polyship.