[welcome]
That being said, test your water. Are these new urchins? How did you acclimate them? Losing spines is bad and usually a result of trauma or severe stress. Three in a 20 is also most likely way too many to keep them happy and fed. If your water parameters are out of whack that could be the root cause. Or, if they're new, a poor acclimation will do it. Stars, urchins, shrimp, etc... should be drip acclimated. Need some more tank info to help direct you here.