i am a mother of 3 months old baby

his poop is green in colour sometimes. 

is it a thing to worry?

If your baby on Bf then its normal, it happens when baby takes foremilk (initial part of breastfeeding, watery milk) and don't get sufficient hind Milk (contains fat which helps baby to gain weight) Just ensure baby latch propely on each breast for 15-20  minutes so that he/she will get hind Milk .  Don't worry feed ur baby in every 2-3 hours.

No it's ok. Infants sometimes do rainbow potty black Yellow green... Don't worry

Black, Green, Yellow - Your Baby’s Poop Story no need to worry.  go through this article. 

he is taking powder milk no breast milk