I would do anything for my boys but I can't forgive her for what she has done (the cheating, the lying, the blaming, and the crap!).
It is b/c you are so angry at "her" and you are talking out of a lot of emotions that are spilling all over the place. You need to figure out how you can love your S3 and not tie him into the bad feelings you have toward your W. It would be better for you (and certainly for S3) to not think about how he could possibly be OM's child b/c if you don't figure out how to divide those feelings, you will think about the A when you look at S3...and you don't want it to be that way. Neither do you want to make a difference between your oldest son and the youngest. They are so samart and if S3 detects any coldness or bitterness from you, he won't understand why and will simply think that you love his brother more than him.
I always admired a woman who could give birth to a baby she concieved when raped. It seems that every time that mother would look at the baby it would remind her of that horrible experience, but somehow she overcomes the bad and is able to love that which is innoncent and good.
I pray that when you look at your sweet little boy's face--you will see that which is innoncent and good and never relate him with an affair. There was a younng man who use to be here on the board and his W had an A and then she discovered she was pregnant. To make a long story short, he DB the M and he chose not to have a DNA b/c he wanted that baby and chose to make the child his. The baby was born and looked just like him (lol) but he still didn't have the test. The M was doing well, but the point was that he "chose" to make that child--his child.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!