Question time /tg So this guys ability has had me stumped for

>>54896949
three. They all trigger seperately.

The three Heralds are separate cards and you would apply their effects separately for each spirit that enters.

So three.

>>54896949
Whenever you see a card's name on a card in magic, unless it specifically says 'a card named WHATEVER', it always refers to that particular instance of that card alone and can be replaced with the words 'this card'
With three copies of Nebelgast Herald, you have three copies of 'Whenever this card or another Spirit enters the battlefield under your control, tap target creature an opponent controls', thus each one triggers when a Spirit enters (including a fourth Nebelgast Herald, which will trigger itself as well.)