Most likely just be ur babies body getting use to it. Hypoallergenic formula does usually make there poos deep green so it's most likely ur babies body just transitioning from milks bit of the old milk and the new milk in there. It takes anywhere between 2-6 weeks for body to get rid of all the old formula or breastmilk used previously x
Colour is normal and nutramigen usually causes slightly runnier poops As its only been 3 days there won't be any telling if it's helping yet as can take 6 weeks to help