Can one mans sperm dominate anothers? I'm 3wks preg, had sex with a man other than the babys dad, is it possible for hid sperm to dominate the dads making the baby his?

?? Dominate?? The sperm that gets there first determines the genes passed at conception. Once that is done, it doesn't matter how many men you have sex with, the one that provided that initial winner is the dad.
This. This is absolutely impossible. If you are already pregnant then the sperm and egg have already combined to form the zygote (fertilized egg). There is no way any other sperm can penetrate and "dominate". On another note you say you are 3 weeks pregnant. Are you sure about this, by the time a woman misses her period she is about 5 weeks pregnant. Please see your doctor to confirm your pregnancy and check your dates by your last menstrual period. Best of luck.