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.