Another picture

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Well it's not an Oxalis which is very good news. Looks like a true 'clover' of some kind but it isn't possible to tell which from these pictures. We need better focus, a scale and preferably flowers. Do I see black markings on the leaves? Or are there any chevron marks on them? Were there any clovers in the wild flower mix?

Here is another picture - not sure how much it will help you. It is possible there was some clover in the wildflower mix I used, but I don't have the bag anymore and don't remember for sure. And no, I don't see any chevron marks.

I'm mainly wondering if I should kill this now before the growing season starts, or if it is probably ok to let it grow and see what happens with it. Thanks.

Closeup

Well - I confess I'm a bit puzzled. It does look like a Trifolium species but not like T repens or T pratense, although it could be T repens which doesn't always have chevrons. I don't know N American Trifoliums. Personally, I'd watch it carefully and see what the flowers are like. It might be part of your windflower mix.

This may be a Medicago.

Update: I believe I figured out that this is Crimson Clover. I found an old seed bag and that was part of the wildflower mix I planted here...

Keep in mind that many of what are called "wild flowers" are also called "weeds". A "weed" is simply a plant you do not want growing where you do not want it growing. So Crimson Clover could be a desirable plant sometimes and a "weed" other times as could be a rose.