see the wiki on monopolar or guyed monopolar... 

https://wiki.openstreetmap.org/wiki/Key:design#Towers_or_pylons

Mostly used as an anchor tower at very high voltage and/or long spans. This tower type is typically used in groups of three, with each carrying one phase. There are two recommended options for mapping such a group. Either map all three towers and connect the power line to the middle tower. Or map only one tower and add the attribute triple_tower=yes to the tower.

No recommendations for splitting out the phases and noting Circuit = 1/3