The smart switch must be on a live line wire otherwise once it was turned off the "smart" part of the switch would stop working. In 3/4 way switches - there is only one "main" that is connected to the line and that is where the smart switch must be (also note, not every smart switch is 3/4 way compatible). Different smart switches can handle different configurations on the other switches. Many require a companion switch or some sort of jumper wiring to be done. Innovelli is the only one I've seen reliably work with existing dumb switches reliably w/o tweaking. If there is no line coming into the 3 or 4 way box via traveler, then you'll have to buy a battery powered remote that pairs with the switch (Caseta has one, I'm sure there are others).
The smart switch must be on a live line wire otherwise once it was turned off the "smart" part of the switch would stop working. In 3/4 way switches - there is only one "main" that is connected to the line and that is where the smart switch must be (also note, not every smart switch is 3/4 way compatible). Different smart switches can handle different configurations on the other switches. Many require a companion switch or some sort of jumper wiring to be done. Innovelli is the only one I've seen reliably work with existing dumb switches reliably w/o tweaking. If there is no line coming into the 3 or 4 way box via traveler, then you'll have to buy a battery powered remote that pairs with the switch (Caseta has one, I'm sure there are others).