My 3 year old has bright green poop. should i be concerned if it lasts more than a few days?

Maybe.: Color change in stools can be the result of many things: diet, illnesses, and then the last category: who knows. Sometimes a kid's stool changes color for a week or two then changes back, and the kid is just fine. If your child has a fever or looks really uncomfortable, stomach pain, vomiting, etc. Then see your doctor. Otherwise, it's not urgent and give it another week.

Green is normal: The body adds bile to the digestive process that colors the poop.It starts out dark green & gradually changes through green, yellow & brown as germs make metabolic changes to the bile.Green is a normal stool color that can infer things are moving through quickly.If other things make you think the kid is sick, get checked, for this, ignore it.