Could it be yellow sedge? If you pull on the individual plants, does it come right up?

They are somewhat easy to pull up, heres a picture:
{{gwi:210296}}

Well, it's not sedge, for which you should be glad,but it still looks annoying. I recommend that you take your beautifully clear pictures, including the last one, over to the Name That Plant forum here on GW and let the wizards of botany over there tell you what you've got and how to get rid of it.

Or, if you've got kids, get them to pull up each and every plant, and pay them per bag full. Don't tell them who gave you the idea, though.