Ambien( zolpidem) is used for sleep induction.
If overdosed, it can cause depressed respiration, excessive sleeppiness, pin point pupils etc and if severly overdosed, it can cause death by cessating respiration.
Now if someone takes more than 5 times of suggested amount( at least 5 pills of 10 mg) will be considered overdose.
But overdose of ambien and its effects also depends upon several factors like , concurrent intake of alcohol, tolerance developed, body weight of the person( more body weight, less effect of dose)
Taking 3 ambien of 10 mg in this 12 years old kid is dangerous.If he is feeling excessive lethargy, difficulty in waking him up, you must call 911 immediately