Spikes III is meant to give wither for 1.5 seconds but it does no damage when people hit me. I suspect this is a bug but it may just be because the wither is too weak to hit within the 1.5 seconds.
it is because in 1.12 the protection enchant on armour protects against potion effects unlike 1.8. therefore 1.5 seconds of wither is not enough to damage. however if you try it with spikes 4 it does do damage. hope this helped