Slender single stem? Check. Three mottled green oval-shaped leaves with pointed ends? Check. Flower parts in threes, including maroon petals? Check. Popping up in early spring in the Missouri Ozarks? Check. All these clues point to this being a member of the genus Trillium--and it is! Specifically toadshade (Trillium sessile), a spring ephemeral wildflower commonly found throughout the Ozarks, the Ohio River Valley, and the central Appalachians. (Technically the stem is a scape, and the leaves are bracts, but you'd be forgiven for just calling it like you see it when casually observing this species.)
If you want to learn more about what traits to look for when observing a new-to-you animal, plant, or fungus, and then using those traits to identify your mystery species, my forthcoming book, The Everyday Naturalist: How to Identify Animals, Plants, and Fungi Wherever You Go (Ten Speed Press, June 2025) walks you through my identification process that I've developed over the past two decades. No experience required, and it's not specific to any region but can be used anywhere! Ask your local bookstore about it, or you can preorder a signed copy from me at TheEverydayNaturalist.com