A better choice would be something like 12 V or 15 V. This leaves plenty of headroom for the resistor so small inaccuracies in either part do not significantly alter the LED brightness and optocoupler output current.
If a simple go/no-go signal is all that is required, use a 6.2 V zener for all three circuits, with different resistor values for each so all three LED currents are approx. equal. Commonality of parts usually is better, and three different zener values cost more than three different resistors.
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