You say that neither your friends nor your family are very understanding when it comes to sexuality. If you think you would be in danger of violence or of being forced out of your home, I would suggest that you not come out to your parents until you have achieved some level of independence from them. If you think it is likely that your parents will react in a way that will endanger your life, health or future, you should work on your independence and also on finding allies who might be able to help you.
It’s always good to have an ally or two when making a difficult revelation to those around you. You already have your therapist. You could consider bringing one or both of your parents to a session with your therapist. This will allow you to reveal your feelings in a safe environment, with an ally present. Your therapist will also be able to help you answer any questions your parents might have and could possibly help them deal with their first rush of emotions.
Within your group of friends, is there a person who might be more accepting than the others? You do not have to reveal yourself to all your friends at once. If there is one that you believe will be more supportive, try talking to them first. They might be able to offer moral support and advice when you decide to tell others.
You can also consider finding yourself a second group of friends from within the LGBT community. Other trans or gay teens might have experience in common with you. Others who have gone through or are going through similar experiences to yours can provide you with support and can prevent you from feeling alone or rejected if some of your friends do not react well to your coming out.
Remember that you are not obligated to tell anyone about your feelings if you are not ready to. The pressure of gender dysphoria can sometimes be overwhelming, but it’s important to balance your desire to change your body with the practical considerations regarding your family and friends.
There’s a time for everything, and although this has probably been a trying 3 years for you, when the time is right you will know, and others will take the news as they will, no matter what you choose is the best path for you. Remember that it is YOUR path.