Melt 3 Tbsp. butter in a 10-inch ovenproof skillet over medium-high heat; add chopped zucchini, squash, and onion, and saut 12 to 14 minutes or until onion is tender.
Remove skillet from heat.
Whisk together eggs and next 3 ingredients until well blended.
Pour over vegetable mixture in skillet.
Bake at 350 for 33 to 35 minutes or until edges are lightly browned and center is set.