That's an interesting question. I have an asymmetric 3 way point, so if the point is set to the first turnoff, it doesn't matter what the second one is set to.
Going back to first principles, a 3 way point has three possible exits, so whatever selection mechanism you put at the front end, it has to offer 3 possibilities. These 3 possibilities are provided by a control mechanism with 4 possible combinations. Even though one of these combinations is redundant, you still need its active partner, so you have to provide 4 control signals.
On a traditional control panel you would probably use a 3 way switch and use a diode matrix to activate the appropriate motor coils for each of the three options.
On DCC, I suspect that at the sharp end, 4 control signals have to be supplied. The equivalent of the diode matrix would be some route setting software in the cab. Some advanced cabs may even have anticipated 3 way turnouts and come with it already built in. I think the ECoS might provide this option.
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