My 3 year son passing urine very frequently and says has burning sensation?

Bladder infection...: Your son's symptoms are very suspicious of a urinary tract infection (UTI). He needs to see a doctor TODAY and have his urine tested. May likely need antibiotic. Make sure you take him to the doctor with a full bladder so obtaining a sample wouldn't be so difficult. There are other non-infectious reason for his symptoms but very important to first rule out UTI so it doesn't spread to kidneys.

Dysuria: Complaints of burning while urinating are often caused by urinary tract infections or urethritis (inflammation of the urethra). A doctors office can help you sort this out with a simple urine test.

Have him evaluated: I recommend having him evaluated by his doctor to make sure that he doesn't have a urinary tract infection.

Needs urine checked: Burning and frequency are 2 common symptoms of a urinary tract infection. As long as he is otherwise well he should be seen at the urgent care for a urinalysis and a urine culture and likely started on antibiotics.