Answer: It's pretty early to plant cucumbers, zucchini and summer squash, they are all related (they are cucurbits) and these need warm soil and do not tolerate any frost at all. But, you are in luck because all three of these look pretty distinctive as seedlings. They are larger scaled seedlings at the seed leaf stage and then their true foliage is large, it looks just like the adult plant. In a quick search I found a photo of an uprooted cucurbit seedling, it shows the seed leaves and true leaves. (It had a disease that affected the stem but ...) You may need to cut and paste the complete url into your browser to make it work correctly.
You may want to try again by starting them in individual containers and then setting out transplants so you can see what you are working with more easily, then next year you will recognize them immediately. Also, you may want to mulch in between the seedlings to keep down the weeds. I hope this helps.
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