As far as I know, this is a wood-sorrel, aka Oxalis, perhaps an oxalis articulata in this case? I think I might have needed to wait for the flower to bloom to know for sure. If anyone can id them properly, let us know

Lawn owners usually hate them around here, as they only want to see grass in their lawns, which means I only pick them where I know herbicides/pesticides aren't used (and hope there isn't runoff from somewhere else)