This question comes up frequently. Some people say you will have no trouble adding the extra person when you check in. I have personally never tried it. But when I check in for an ADAr on the app, it will sometimes give me the option to add another guest. Note: SOMETIMES. It will often give me a message that I can add “0” extra guests. It seems to be dependent on the restaurant’s capacity when you’re checking in. A popular restaurant at the busiest meal times you would probably not be able to add. An average restaurant at off times, probably very good chance. Not sure what your chances will be.
You can try to make another. ADR at the same time, for 2 people. Then check in at the podium, not the app. Explain you’re together with the other ADR & ask to be seated together or as close as possible. At least that was you could sit 2 & 3 if you have to.
Also, keep checking. As your trip gets closer & especially the day before your ADR. Lots of people cancel ADRs the day before as their plans change. Good luck!