Here's a third vote for Oxalis. Like Baa said, there are so many kinds and some can easily be invasive. A neighbor has some lining her shade garden beds ~ little pink blooms about 1/4" across and leaves 3/4" to 1". I have some that are growing wild under the trees in the pastures ~ haven't seen a bloom, but the leaves are easily 1 1/2" or more. I don't know if it's native/indigenous or escaped and naturalized, but they've been here forever.
Be very careful with that one, this is my #1 WEED!!! Gives me the biggest headache of them all, it not only seeds itself, but makes dozens of little bulblets each year which drop off if you try to dig it up. Don't let that one get loose in your garden or you might regret it.
It's Oxalis latifolia, from South America I believe.