Question

unknown plant   This plant appeared about a week ago and continues to spread.  It's about 3 inches tall and covers about a 15 ft long by 4 ft wide section in yard.  There is a few other places where it is just a little section popping up in other spots.

I live in Missour, outside St Louis.

Answer
Karen
Plant types vary from area to area...Plants that grow in Missouri do not necessarily grow in other areas of the country...Flowering plants are classified by the structure of their flower...I have checked numerous plants in your area but saw none with the leaves you have in your picture...Take the picture to your local garden center for them to ID it...You may have to visit more than one to find someone knowledgeable of the plants in your area...

Good luck
Rick in southern NJ