Well, dear, three crosses are a symbol of a very important event in Christianity. In this event, a man named Jesus was punished for things that he did not do wrong. He was hung on a wooden pole called a cross as a punishment, along with two other people.
One of those people made fun of Jesus, but the other one asked Jesus to forgive him and showed that he belonged to Jesus. So, when Jesus died, he promised the second person that he would go to paradise with him.
Christians use three crosses to remember this event and to show that Jesus died for their sins. It is a reminder to them that they should always try to be good like the second person on the cross, who asked for forgiveness and believed in Jesus.