I combine 3 IHG free annual nights every year. I book the first night in my name using my cert, 2nd night in my wife's name using her cert and the 3rd night in my dads name using his cert.

My dad does not travel so I use his points, miles, free nights etc.

Some years I can add myself as a second person on nights 2 and 3 and some years I can't add me to the other reservations. For April 2018 I was able to add my name

Regardless of if I can add myself to my wife and dads reservations I call the hotel about 3 months in advance and tell them we have 3 nights and would like to stay in the same room. I then callback again a few days before we arrive.

Upon arrival I have all three reservations printed out and explain the situation again at check in

I have never had a problem.