They're all fine grammatically, but "life's uncertainties" is the only one I've ever heard anyone actually use in real life.

Probably "life's uncertainties". What is the full sentence?

The most natural and commonly used expression is "life's uncertainties." It's concise and flows well in conversation or writing. However, all three expressions are correct and can be used depending on the context: * "The uncertainties of life" – This is formal and slightly philosophical. * "The uncertainties in life" – This works, but it's less common and may sound more specific, as if referring to particular uncertainties. For everyday use, "life's uncertainties" is the smoothest.