Which Neutral for a Single Pole Leviton Dimmer?
The neutral is the bundle of white wires.   

The "3 hot 1 neutral" is not a neutral at all. It is a white wire which has been repurposed as a hot.   Think that through: a case where color does not depict function.   

It's not the only one: the line north from the 3-way is a switched-hot that I prefer to mark red since it's switched-hot, and the red-black pair leaving the 3-way are messengers I prefer to mark yellow. 

Code, however, does now require you mark your re-purposed neutrals.  Before it was allowed to not mark them if the usage was obvious.  Which was a pretty editorial threshold, as you see!