For the Vinaigrette, mix all ingredients in small bowl with wire whisk until well blended. Set aside.
Trim squash ends. Slice yellow squash lengthwise into ribbons with a vegetable peeler or mandoline, discarding outside ribbons and core. Slice zucchini crosswise into thin round slices with a knife, vegetable peeler or mandoline. Slice carrots lengthwise into ribbons with a vegetable peeler or mandoline.
To serve, divide squash and carrots among each salad plate. Top with radishes and onion. Serve with Vinaigrette on the side.