A few things I'd toss into the mix:
Boxes of outlet covers are cheap and three year olds are still curious enough to poke fingers. I'd get enough for the rooms they'll be in.
I'll second having a stepstool in the bathroom. Little ones often can't reach the sink.
While you're moving cleaning supplies, remove anything from the bathroom that would break your heart if spilled. Little ones sometimes get into shampoo and soap bottles too. Also, remove precious breakables from the whole house since even well-behaved kids sometimes tip things over.
Set aside a can for soiled diapers/pull-ups if the youngest is still in for that, or have the collection can convenient. You don't want them in the kitchen trash. Trust me. Having a box of baby wipes at hand can help too, since sometimes parents run out or forget. You can always run out for these, though, if it becomes an emergency.
I'd personally go for crayons before washable markers, because they can't draw on furniture or themselves as readily, and "washable" isn't as washable as they promise. Be careful of washable crayons, though, since they dissolve in hands and when they get wet.
Beside all that, it sounds like these kids are going to have a ball. Good for you to go out of your way to make them happy.