This will sound harsh and like I have no heart BUT.....
THEY chose to have 3 kids. It is up to THEM to support those 3 children and that means paying for daycare. Why should you give them a break when you could take on 3 different families with one child each and be making your full amount?
There are pros and cons to taking on families with multiple children (the biggest con for me is when they leave I'm stuck filling 3 spots as opposed to 1) and it is ultimately up to you to decide what you want to do but for myself, I don't offer sibling discounts. It wasn't my choice to have all those children so why should I be the one loosing out on pay because of it.