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I am extremely unhappy with this circuit. If only one phase is active, then only one opto is on and only one transistor, but with your component values, the output will be a '0'. Well actually a '.5' because of the 1k emitter resistor. I would try putting all three transistors in series, no emitter resistor and proportion Rc so that with all three phases present, the Vc of the 'top' transistor is .6V (3 X Vsat). Then if a phase goes missing, the voltage at this point will rise. I do not think your series resistors are high enough in value, I feel 1M would be safer.