they can't charge you something that it isn't indicate during the booking.

Each rate on IHG shows the room taxes, resort fee /whatever and if applicable "extra change" for adult.
As far as you can see the "100.00 USD per extra adult starting with the 3rd adult. (not included in price)" is only on the best flexible rate.

On the award booking and on the advance purchase rate there isn't the extra change for the 3rd adult and I'm sure there isn't any mention about it on your confirmation e-mail.

Don't pay it and fight it