You are a married women and know what is the character of man. He made a mistaken 18 years ago and choose to stay with you. It means that you are still his top priority and you should value it.
On the other side, the 小3 is the person also and she suffered from this. Her kid is your hu*****and's kid and he loves him/her also. If you can excuse your hu*****and and even discuss with your hu*****and on how to help 小3's kid, your hu*****and will love you to death and you will have a happy family.
You will lose pretty much everything if you divorce and your two kids will surfer also. Be a great women and then you will have everything you want!!!