> If your property has 3 phase power, chances are your single phase breakers are just on some random phase (picked by your electrician) ie phase 1.

That's bollocks or speaks for a completely incompetent electrician. Usually, you have three phase rails coming out of your GFCI that distribute the phases sequentially to each breaker [1] to avoid that scenario, as well as ending up with a tripped main breaker because the customer overloads the phase.

The only case where great care is being taken to avoid random assignment of phases is in event stage technology - you do not want your lighting dimmer packs on the same phase as your amplifiers because dimmer packs inject extreme amounts of EM noise, and you also have to take care to not put overloads on any single phase anywhere. However, it's questionable on how long this will stay relevant, given that most lighting load is moving off to LEDs.

[1] https://www.amazon.de/-/en/Busch-Jaeger-Hage-Phase-KDN380A-3...