Small trifoliate volunteer in the uncultivated portion of yard. Blossom is about the size of a half-dollar. Before it blossomed I assumed it was clover, but was I fooled.
this is Oxalis rubra, the windowbox woodsorrel, a commonly cultivated and escaped species often mistaken for shamrocks or clover. Yours is a larger than usual cultivar. It has an underground corm, looking like a small bulb, and can be propagated by these. A similar species is the native Oxalis violacea.
I hope this helps.
Steve Hill, Botanist, SCNPS