That happens to a lot of people..your heart rate
accerates and you can feel it in your throat... As far as normal heart rate for 3 year old boy: 60–140 beats per min (at rest) (per webmd.com) Your cardiologist will know, call him and find out for sure.
When it comes to conditions with the heart you should always get a second opinion
(most insurances will cover this too). You can wait till after the visit in 3 months or you can get a copy of his records and see another doc now... which ever one will help you to relax and feel better. I wish you and your son well. I understand your concern: My family has a history of heart defects and its scary to think that my children could suffer from them too. Take Care and do what you feel is the right thing to do. Trust your instinct as a mom.