Perfectly normal. It's amazing how aggravating these chickens can be.
I've had pullets lay their first egg at 16 weeks. I've had some wait nine months. Nine long slow agonizing months. She did not pay any attention to a KFC advertisement in the paper, even when I discussed alternatives with her.
Each one is an individual and will lay when she lays. The more you have the more likely you are to get an early layer. With three that age it "shouldn't" be that much longer but who knows. I wish you luck!