1.first decide thoroughly if you really want to add all 3 PASSes on 1 fone-or not. If you chose yes, you have to travel together all the time, you cannot move it to other device once installed-obviously anti-fraud measure. If you choose no, this means that anyone has to activate and register trips for themselves but still for same trains.

2.again; RES and triprecord are separate from one another. It is often even better to do that RES via other ways as app. IF you make 3 RES in 1 time, most often the systems will try to put you together-even more imprtt if also nighttime slepper etc are wanted. Quite often noone will even want to see this RES-it is the PASS=ticket to ride, that is the imprtt thing to be activated and valid when conductors come to check.

Smarter people also think about routing and try to use trains that do not want you to make these pesky RES so often. IN well over 95% of all trains running in EUR you do not need to.

You don't book seat reservations in the app. You can book them through the Eurail reservations portal on the website, but there often are better places to book them. See this list for more information:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you do want to book them through the website for various travellers, you can add travellers in your account, under "reservations" and then "Edit Your Trip and Travellers" (https://www.eurail.com/en/my-account/reservation-booking-details/trips-and-travellers not sure if this works)

Once you've done that, you can book seat reservations for all passengers you have added through the reservations portal (https://www.eurail.com/en/book-reservations)