I agree nothing to worry about. Late teething is usually hereditary. Ask both sets of parents and you might find the answer.
My first child got her teeth late and lost them late. she hadnt lost any teeth when she turned 7! and she was told by the dentist she had great teeth recently
The fact your child has three teeth already rules out any of the medical problems that can cause late teeth. Besides there isn't a lot you can do to convince teeth to come up.