L.L.
There is no rule that you have to have an equal number of anything, especially flower girls. Make the oldest the "head" flower girl and she can carry a bouquet, let her walk in first. Then the two littler girls can walk in together carrying baskets with flower petals if the location you are getting married allows them to be sprinkled. If the location does not allow live petals to be sprinkled, ask about artificial flowers, you can get those petals for the same price if not cheaper than the real ones. Congratulations!