J.W.
We are in the same situation with 2 kids, and from what I have learned from many people who have babysitters in high school is the going rate tends to be between $6-10 an hour depending on how many kids you have. Sounds like you start at $6 and add $2 for each additional kid, and although this sounds like a lot, I was a babysitter my whole life and nanny while in college, and I was only making a dollar less back then (about 15 yrs ago) so I would say with 3 kids between $8-10, but I would also just ask that high school student what they think if fair, because sometimes they have different expectations, too, and then come to an agreement with that person. That is what we plan to do...
Good luck and have fun on your date nights,
J. W.