Looks like nut sedge and tough to kill. There is a small nut on the tip of the roots and when you pull the plant, the nut grows a new one. There is spray you can use but it kills the surrounding plants also. I have some in my flower bed and will lay layers of newspaper or the fabric down around my main plants, then mulch on top. It doesn't like the shade.

Nutgrass/nutsedge is a swamp plant. It does best in wet soil. Lots of rain improves the possibility of nutgrass getting started. Allowing the soil to dry out completely between watering will reduce the possibility of it getting a good foot hold. As Emma said, it is hard to get rid of. The sprays are called Sedgehammer and Image. I do not believe they will kill the grass if you follow the label directions.