it is a condition called fasciation and is not uncommon on many plants. I've had daisy, delphinium, veronica all doing it one year then not again.
It is due to disruption at the growing tip when the instructions for growth becomes damaged. possibly insect damage or even frost damage.

Cellular differentiation in plants (as well as animals) is a very complex process involving the internal cellular environment as the plant grows as well as external environmental factors. I consider it as miraculous that every cell differentiates and winds up in the right place and at the right time in the majority. So what you describe is most likely a “ one off”

Yes, I've seen it twice - once this year but with only two heads not three.