Yes you are right, that's quite a simple schematic, doesn't need any micro controller, just simple relay's, the phase loss is detected by the voltage drop between the phase and the NULL, if one relay(LS2) gets a voltage under his threshold(about 150V, depends from the relay), the motor is disconnected.
It works like this:
Start sequnece: SW2(ON) is pushed, it activates the main relay(LS3), each of the LS2 relay's are powered(if all 3 phase are present), the circuit closes, and the main relay state is latched(like a bistabil).
Stop sequence: SW3(OFF) is pushed, this interrupts the circuit if the main relay coil, and the motor is disconnected, if any of the 3 LS2 relay's coil get a voltage under there threshold, again the motor is disconnected(logical AND connection), as you can see, one of the LS2 relays seems redundant, but the main relay has a larger magnetic hysterezis, so this way the symetrical sensitivity of the circuit is kept.
This circuit can be modified to be used without the NULL line, but the majority of the relays got 220V coil (phase-NULL voltage).
My question is: Why you want to use a microcontroller based monitoring, isn't that like killing a bug with a riffle? I mean the phase angle is 120, it doesn't change, the worst case is when you have a low voltage on either of phase, or it's interrupted. For increased sensitivity you could use some electronics for the voltage monitoring/relay command, this way you can set the threshold voltage to 200V, or monitor the voltage between the phases, it is lower than 380(or higher) then it is out of phase, then a window comparator is needed, anyway the microcontroller is to complex for this simple task.
I hope my suggestions are helpful, but you can ask if not
Regards