At 3 days, chicks are just getting started on eating, so some chicks may not poop much, if at all yet. Their droppings are so tiny at this stage, you might mistake them for something else, too, if you can even spot them.
Keep checking butts for clean or clogged vents with poop buildup, and clean gently when you see any poop, no matter how tiny. Watch for signs of lethargy and inactivity. A chick that isn't pooping will not be active, may be standing still and nodding off, or if it's constipated, you will see it squat without producing anything, peeping madly since it hurts.
In the absence of those behaviors, try not to worry.
