Answer:
Generally, there is a fixed "by the hour" rate for babysitting... regardless of how many children are in the family.
Another Perspective
A smart childsitter will adjust their rates to fit the situation. It involves more work and responsibility to care for three children than to care for one. Ask around to get an idea of what other sitters charge and then figure out a rate schedule. Be ready to give a price when someone inquires.
For example, you could charge a base rate of $5.00 per hour (but check the rates in your area) for one child and then add $2 for each additional child. Then, when you get a job do it really well. Follow orders to the letter, clean up after meals and pick up after the children go to bed. Make certain the house is neat when the parents arrive home. Your good work ethic and diligence will be noticed and you will get good references. You should find that when you do a really good job you can charge a little more.