My 3 yo just had a massive temper tantrum. he is now asleep but i listened to his hr. it seems like it slows down than speeds up. worry?

Yes: Your 3 year old has learned how to control you. That is something to worry about. You need to regain control now. Sometime you need to ignore him when he is like that, you need to put him in his room and shut the door. Kids that learn how to control parents will generally become inmates in jail in their late teens and twenties.

Variable breathing: It is normal for your 3 year olds heart rate to vary depending on whether he is breathing in or out. This is because when you breath in the heart gets squeezed a little by the lungs and this slows the heart down. Then when you breath out, the heart speeds up a little as the air goes out of the lungs. This is a normal response and often exaggerated in little kids!