I would really try to eliminate the third person and just keep it grandma and the new sitter. That would just make it less confusing for the kids. Either way, write up a chart that the six year old can read and put it on the bathroom door, so every day that goes by (person by person) he can have sign off on that blank, so they will all know how much longer they have of "freedom!" (you might put that positive spin on it, girls yes, boys maybe...) That way, they can gauge time by people and it can kind of be an official sign in and sign out thing that the six year old can be in charge of keeping.
Also, I would have the new babysitter over all day Friday or Saturday just to watch you or hang out or you could watch her. It might be very worth it. You might spend one hour writing down a tip sheet listing what makes each one ticked and what makes each one happy.
Insurance cards are a must...
Just some thoughts...
Oh yes, have each boy give you something to take from them for you to remember them by and take a picture with it and send it to them via e-mail...it was my idea before the commercial came out...