You don't just reset it. It is an error code & is telling you there is a fault. Fix the fault and the error code goes away. U3 means it thinks it has filled too quickly. Either because there was still water left in the drawer from previous wash or something is wrong with the rotor under the spray arm. Check that tub is empty. If not then find out why it is not draining. If empty then check rotor spins freely by hand. Next remove rotor and check lower blades for damage or bits of rubbish.
Lincoln Appliance NZ, March 2013