Can A Women Sleep With Two Men Three Weeks Apart And Their Semen Mix Together And Make One Baby?

NO.

Only one sperm can fertilise each egg.  The egg ensures this; if two sperm somehow got in there together there would be too much DNA (genes) and the egg would never divide properly to become an embryo, and later turn into a foetus.  It just wouldn't grow at all.

Two sperm can't mix together to become one new sperm.

A DNA test will prove who is the father of the baby.

In theory tests can be done before the baby is born, to identify the father. But these are invasive tests (e.g., amniocentesis), and run risks for the unborn baby.  Tests on blood taken from the mother during pregnancy can sometimes identify the father -- but not as reliably as amnio or a simple blood test after the baby is born.

Nope

I can't find anyone to confirm this but my sister slept with her boyfriend and ex husband in the same day and her son looks like both of them. She claims its her boyfriends but her ex had twins with his girlfriend and one of those boys look like my sisters son. I don't know what to think and neither does her ex.

I was kind of in the same situation. I broke up w/bf and was in a short relat. Then went back to bf and found out i was pregnant. Me and bf got a paternity test done recently and found out  my son does belong to him. My daughter looks like my ex bf and my son looks like the short relationship guy. My son looks just like the other guy i was with for the short while, but the tests says other wise.

No, only one could be the father, sperm could not survive three weeks, but if the first guy impregnated the woman the second guy could not impregnate her at all.