I am with you, skinnypuppy40., but I can see the problem. We do reach a point when we cannot cope with several grandchildren at once on our own. At a certain point my parents suggested that both my children (the only DGC) come and stay separately because they found having both staying was too tiring, and that is what they did, but I only had two GC so they both stayed on their own with their GPs.
When there are three and one is the other sex, then things get difficult. I am from a family of 3 girls but my youngest sister was 5/7 years younger than the two eldest, and we were very close in age, so inevitably we sometimes did things on our own without the youngest. She had her special occasions and treats, but what she remembers, as an adult, is being left out and that has made our adult relationship very difficult.
In your case, the third and youngest is the opposite sex to the two older children and this will increase her sense of being left out. So I understand your problems of not being able to cope with three at the same time but I can understand their father's concerns
But I think there is a very simple solution to your problem. Have each child to stay with you on their own. That way each child will be treated exactly the same and if your grand daughter could choose whether she wants to stay first, second, or third, it will alleviate any feelings of exclusion.
Dealing with one child at a time will also be less hassle for you.