Why would a 3 year old have heart rate of 170 when he is sleeping?

It is not normal for a 3 year old to have a heart rate of 170 when they are sleeping. Normal heart rates for children are typically between 70-120 beats per minute. If a 3 year old has a heart rate that high, it is important to seek medical attention immediately. There may be an underlying medical condition that needs to be addressed.