$60 a day in this area is a pretty good rate for 3 kids.
I would suggest that while its great your doing this make sure that she knows your doing it when it fits for your schedule. Don't get sucked into doing it not only for free but also not being able to say I have something else going on a specific day.
And yes she should be sending lunches and snacks. If she wants them to be doing any activities she should be providing the money for that. And if it is something she wants you to take them to then she should be paying your way in as well.
Out of curiosity has she offered to pay you at all? If she hasn't at least made a token offer to pay then she is taking advantage of you. It is one thing for you to say don't pay but it is completely another for her to say hey can you watch my kids for free I don't want to pay for daycare.
As an FYI I nanny for a family 2 kids that I hike back and forth to all their activities, feed them breakfast and lunch, make sure they get baths etc each day. I have them aprox 10 hours a day through the summer and make $120 a day for this. I live in oswego.