Some of the best things in the world comes in threes.
Look at children's stories. Look at Goldilocks. She goes to the Bears' house, tastes three bowls of porridge, sits in three different chairs, and lays down in three different beds. Of course, when the three Bears come home she jumps out a window in fright, but aside from that, the Rule of Three plays out beautifully.
Look at the three little pigs. They each build a house - one of straw, one of wood, and one of brick - and the wolf comes and huffs and puffs on houses one, two, and three... even though he doesn't knock number three down. Again, here's the Rule of Three.
Look at Jack and the Beanstalk. He goes up the stalk three times before the giant chases him down and Jack is able to kill him. Three again.
My favourite things in life come in groups of threes. God, family, and books.
Food comes in three stages. Preparatory, (chopping, mincing, blending, etc.) cooking, and EATING! :)
Take a look at plants. One of the most famous plants, the clover, has three petals.
Look at the virtues. Faith, hope, and charity are a group of three.
Families are arranged in groups of threes. Father, mother, and children.
Then my favourite. God is Three Persons: The Father, the Son, and the Holy Ghost.