Does urination after sexual intercourse wash away the left over sperm in the male's urethra?
Yes, urinating will remove any remaining semen in a man's urethra. However, it will not remove any traces of semen that maybe on the surface of the penis. I'm curious, though, what is behind this question.
One day, I had ejaculated sometime before my girlfriend came over. I urinated three times before she and I started to get into it.
Are you saying you put your penis into your girlfriend, but did not ejaculate in her?
Yes. But it wasn't as if I waited until the last second to take it out. It was one of those situations where we both stopped and I finished using my hand.
And what you really want to know is if you could have gotten her pregnant.
It is possible, but the odds are very slim. It would not be due to your earlier ejaculation, but when you got aroused. You start dripping pre-cum, the path to your ejaculatory duct can open enough to allow a drop of semen through, which can mix in with the pre-cum. The odds are low, but not zero.
The real problem isn't the remote chance of pregnancy, nor the more real possibility of disease. The real problem is that you are sinning. "Let us behave properly as in the day, not in carousing and drunkenness, not in sexual promiscuity and sensuality, not in strife and jealousy. But put on the Lord Jesus Christ, and make no provision for the flesh in regard to its lusts" (Romans 13:13-14). I would rather you reach heaven that spend eternity in hell because you want sex with girls you are not married to.