Weed in lawn
This is common lespedeza, also known as Japanese clover, (Kummerowia striata, syn. Lespedeza striata). It is a prostrate summer annual that forms 15-18 inch patches. The stems are wiry. It has dark green trifoliate (arranged in threes) leaves with three oblong, smooth leaflets. Leaflets have parallel veins nearly at right angles to a prominent mid-vein. Its leaves have smooth edges and a short spur at the tip of each leaflet. Flowers in late summer with small pink to purple, single flowers found in leaf axils on most of the nodes of the main stems. It grows very low to the ground and chokes out thin turf.
Since you live in Montgomery County you can only use organic herbicides to kill lawn weeds. I don't think they would work very well on this weed. It tends to grow in poor, under-fertilized soil. To reduce this weed concentrate on improving the health of your lawn.
The following is information about the Montgomery county pesticide law, Montgomery County’s pesticide law bans the use of most synthetic pesticides, including herbicides, insecticides, and fungicides on private lawns, playgrounds, mulched recreation areas, and childcare facilities. Organic pesticides are allowed. There are some exemptions for the use of synthetic pesticides, including for biting insects, indoor pests, and tree and shrub pests. Go to the following website for information regarding the law and organic lawn care information, www.MontgomeryCountyMD.gov/lawns
and a link to our lawn care information, https://extension.umd.edu/hgic/topics/lawn-care