Welcome to PF ismat. It looks like the heaters are being controlled by SSR (which is a silicon controled rectifier, I think) and its ouput is controlled by pluses supplied to the + and - terminals. These pulses could be coming from a PID controller. And K3 appears to be contacts for a relay. Are there other parts of the circuit you are not showing?
The voltage at 4,5,6 is just your 440V line voltage. The voltage at 7.8.9 will be pulsed 440V.
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