1. Three Might Be A Crowd
As tempting as it is to accommodate everyone when the opportunity arises, if you invite additional kids along to the play date, you may be throwing the kids and parents for a loop.

If a play date was going to be 1:1, ask the original person you were going to meet with if it's cool to include more people. This is after all, a "date." 

That said, some kids do better in groups than in 1:1 situations. As with all play dates, to a certain point finding harmony is a total game of chance.