Welcome to BYC. Glad you decided join our flock. Since the chick starter (assuming that's what you're feeding your chicks) does not need the addition of grit (treats do), I would keep the grit in a separate bowl near their feed, that way they can peck at it whenever they feel they need it. There is a good article on feeding chickens at different ages at http://www.the-chicken-chick.com/2012/11/feeding-chickens-at-different-ages.html. Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck with your chicks.
You'll have some nice big fluffy chickens with those 3! If you haven't already done so, read up on integration tips so your girls have the best chance to start off with the others on the right foot. Have fun with your new little ones!