Here's some (in my opinion) great advice:
1. Ask them to have a backup daycare option available, and say you want to try a 2 week trial. Let them know you'd really like to do it, but that you also have to make sure you can handle 3 little ones at a time. Make sure they understand that you have the best interests of the children at heart, and you don't want to commit to it if you feel you won't be able to adequately care for all 3.
2. Make sure you have a payment contract in place, and let them know that this is a business arrangement completely separate from your friendship. I can't tell you how many posts I read about friends taking advantage of another friend's offers of babysitting. You don't want this to become a situation where you're not getting compensated for the work you've done, and it starts to affect your friendship with these folks, and possibly your feelings towards their baby.
Let them know your daily/weekly/monthly rate and tell them exactly when you expect payment. If they start to pay late or not at all, be firm and tell them that you cannot continue to care for their child and that they'll have to find another arrangement by x date. Be firm. Do NOT let yourself get into a situation where you feel taken advantage of. Be sure to put a clause in the contract that either party has the option to terminate the arrangment with x day's notice.
Good luck, and I hope it works out for you.