My baby's poop is dark green for 3weeks now, it doesn't bother me at first because he's seems fine and gaining weight. is it normal for a formula fed baby to have dark green poop?

Poop color is based on the primary additive (ultra-green bile) and the action of enzymes and gut germs on it as it passes. It changes through yellows to browns if it stays long enough. I don't consider this color a problem at any age.

Yes. Green poop in a child is not a cause for concern as long as the child is eating well and gaining weight and is playful. Wish you and your child good health!