Hey there, Carl.
I'm going to pass on a little wisdom my grandfather passed on to me - much to the chagrin of my parents. Sometimes, it's better to ask for forgiveness than to ask for permission.
You will find that there are certain restaurants at
Walt Disney World are super hard to get reservations for at all. Once you have them, it is super hard to change them. The one restaurant most likely to have this scenario is
Be Our Guest Restaurant.
I would advise this - keep trying, all the way up until the day of if you can. Check various times of the day, but especially early in the morning. You may luck into an open reservation for three.
If that fails, go the ask for forgiveness route. Turn up at the restaurant for your reservation, and let them know when you check in that you have three in your party, not two. They will do their absolute best to accommodate all of you. The obstacle is that going from two people to three isn't a big deal in number, but it moves you from a table that typically seats two guests to having to find one that probably seats four. That can be tough.
Don't give up, keep trying, and if they absolutely cannot seat you as a party of three, there will be other options. My gang would hop on the monorail to Disney's Contemporary Resort for a delicious breakfast buffet at
The Wave . . . of American Flavors, but I love my Main Street Bakery Starbucks!
Good luck, Carl!
Anna