If the shed has a mwbc or a feeder going to it and then you install another circuit from the house or vice versa would you not be violating 225.30
It's done all the time does not make it code compliant.
I have done it myself but I don't think it complies.
But I don't think this would apply if you feed the 3-ways from the garage/shed building?
heres what I believe, lets say the light outlet is on the garage/shed, if you feed the switching also from the garage/shed then your not adding any more circuits.
now lets say the light outlet is on the house, you feed the switching from the house, is the 3-way a point on the circuit in which current is taken? I wouldn't think so, so your not really feeding the garage/shed but you can run into a problem meeting the requirements of 225.31.
but feeding the circuit from the garage/shed with the load on the garage/shed would be disconnected and your not adding any extra circuits, and also meet 225.31 with one disconnect.
now put the light outlet on the house, and feed the circuit from the garage/shed, you will now have one circuit feeding the garage/shed, and a circuit feeding back to the house from the garage/shed, so the garage/shed disconnect requirement will be satisfied, and the house disconnect will be satisfied, but will this be considered another circuit running to the house?
I wouldn't think so because it originates from the house to start with.
ok last one, put a separate service on the garage/shed and the whole thing changes, now you have a violation either way if the load and branch circuit is not on or from the same building, again I don't think just feeding a 3-way is feeding a load