I cast 
Song of Blood, and the cards revealed are 
Swamp, 
Grizzly Bears, 
Grizzly Bears, and 
Viscera Dragger. Once it resolves, I unearth 
Viscera Dragger. When I declare attackers, 
Song of Blood's delayed triggered ability is put on the stack. When it resolves, will my attacking creatures get +3/+0 or +2/+0?
My initial inclination is +3/+0, as 
Song of Blood's delayed triggered ability cares about how many creature cards were put in the graveyard when 
Song of Blood resolved.
The reason I'm asking this question is because Magic Online gives the attacking creatures +2/+0, and I can not conclude if this is a bug or me misunderstanding the rules.
Thank you.