The little 3 leafed plant is clover. if you find one with 4, it's lucky. As for killing it, look for a weed killer that won't harm your grass. Once the clover is gone, make sure to plant new grass in its place.
I've had better luck with keeping weeds like that out of my grass by keeping it mowed high
I see people in my neighborhood with very shitty lawns because they mow too low. I mow at the second highest setting on my mower and my grass is consistantly better than anyone else's on my street.