This is lespedeza, a summer annual weed. Click here for our webpage: https://extension.umd.edu/hgic/common-lespedeza-or-japanese-clover
But the real problem is that your lawn has so little grass in it that, even if you kill all the weeds, the grass cannot fill in the bare areas faster than the weeds will grow in again. So, using weed killer is not going to solve the problem.
When a lawn is more than 50% weeds, it needs to be renovated, i.e. killed and reseeded in last summer (late August)-early fall. Here is information to get a successful lawn: http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/lawn_pubs/HG102...