Even without hearing about the girl with the difficult temperment, I know that it's always harder to have 3 kids together, because inevitably one always feels "left out."

I know it's not easy, and you want kids to have time for "free play," but personally I think that having structured activities often helps when you're dealing with "difficult" kids. You might try posting a "schedule" for the day, and try, as best you can, to follow it....It may help the "difficult girl" to understand that whining won't get things to change, and it can help your son and the other girl to realize that whatever activity is bothering the "difficult" child won't last forever. (For example, if the schedule says "Board Games 10am.-10:45 am," and "difficult girl" starts losing and wants to quit halfway through, tell her that she can either continue to play for the remaining time, or else do a quiet personal activity, like reading a book or coloring. You won't change the group schedule just for one person. If your son and the other girl also want to quit, that's a different situation.) Offering a snack when tempers are heated also seems like a good way to calm things down in a group of children. I know some kids seem to get "moody" when they're hungry/blood sugar levels fluctuate. (Plus, when kids are eating, they do less talking/taunting each other.)

When things would get really tense during "playgroups" when my son was younger, I always tried to have a few "special" activities around. For example, making the recipe for "homemade playdough" was always a good distraction. (It's prepared by cooking on the stovetop, using regular kitchen ingredients...I handled the heating, but they loved kneading and pounding it out and using cookie cutters shapes, etc.) If you have to physically position the "difficult" child on one side of you, with your son and the nicer girl on the other side of the table, so be it to keep them apart.

While none of these suggestions is going to change the personality of the "difficult" child, I think your son and the other girl will realize that even if the "difficult" child wants to have a tantrum, they can still engage in fun activities themselves, and nothing lasts forever...thankfully, the "difficult" child does go to a different home at night!