Your 3 yr old is likely to be suffering from Urticaria
an allergic rash
since he is itching and the lesions look like tiny bumps .He would require anti- histaminic like benadryl
and also oral steroid
for a short while to stop getting a recurrence.Also one will have to search for the reason for such allergic rash like dust mites,food,pollens etc. So i would suggest you to get him examined by his docter for further management.