"Happy Birthday Sweet boy! 3 is a magic number" the Oscar-winning mom captioned the photo of her adorable towhead posing in the sand wearing bright blue sunglasses.
Not long after, Witherspoon posted a snap of the lucky tike's birthday cake, a chocolate confection topped with a fleet of miniature construction trucks that would make any young boy drool.
Tennessee isn't the only member of Witherspoon's brood to celebrate a birthday recently. Two weeks ago her daughter, Ava Phillippe, turned 16 years old—an event the proud mom marked in a similar fashion, on Instagram, of course!