M.P.
My granddaughter took Singulair for a couple of years. It seemed to make her more agitated. Finally my daughter insisted that she stop taking it. The doctor said that she had to take it because of her asthma triggered by allergies. The doctor prescribed Zyrtec for her (at that time it was available only by prescription) and her allergies are still under control and there have been no side effects.
I would not stop Singulair,or any medication for that matter, without contacting the doctor. Talk with him about using Zyrtec for a 3 yo or ask if there is anything else to try.
Singulair is out of the system fairly quickly. A pharmacist could tell you exactly how long.