I put my ear to my 3 year olds son chest to listen to his heart. His beat sounded normal for about 5-7 seconds than a pause to the beat than normal?

Yes normal. Many children have irregular heartbeat called sinus arrhythmia. It is normal. It has no symptoms and needs no treatment. If he has symptoms like fatigue or needing to rest a lot...That is not normal for a 3 year old and he needs to be seen.