Not sure if I completely understand the question. Today when I book three tickets together, I pick E+ for myself and one child and E- for the other child. I’m assuming with the subscription, it will know that I get one companion and won’t charge for the third E+ seat.
Most of the time I’ll play dumb and ask “is there anyway we can get three seats together?” And the agents are usually extremely polite and say “no problem” or “I’m supposed to charge, but since your daughter is only 7, I can do it this one time.” I did recently have one agent almost laugh at me for asking the question, “yeah I can move your daughter but you have to pay for the E+ seat.” I assume because of an equipment change, the 737s we fly often have a mysterious row of E- seats appear in front of the exit row about 24 hours before departure, so we’re usually close together. The kids always take the E+ seats and stick me in the E- complaining that they don’t want to ride by themselves :-( I’ll be at about 60K miles at year end, so maybe I should wait and see if I get a platinum buy up offer...