Bad heating element (all three is unlikely, though).
Failed wiring/connection to heating element (more likely for single phase)
Faulty temperature control or probe.
Failed wiring to the temperature control.
Faulty centrifugal switch in blower motor.
Faulty wire connection to centrifugal switch.
Maybe something else that I can’t think of at this moment…
You said NOTHING about which temp control you have – be it a COOK ONLY, a SOLID STATE DIGITAL, an INTELLIPLUS, INTELLIHOLD OR INTELLIRACK, a BLODGETT IQ, an INFINITE WITH PULSE PLUS, a TWO SPEED WITH INTEGRATED CONTROL or the TRIPLE SETPOINT THERMOSTAT. So – I can’t go further in any additional & enlightening assistance….
That’s about it. I hope this helps you in your diagnosis.