Cook broccoli, covered, in medium saucepan in small amount of boiling, lightly salted water for 3 to 4 minutes or just until broccoli is crisp-tender; drain. Immediately plunge cooked broccoli into ice water. When chilled, drain.
In a large bowl, combine broccoli, squash, zucchini and hot red pepper flakes. In a glass measuring cup, combine vinegar, oil, lemon juice and sugar. Pour over broccoli mixture; toss gently to coat. Cover and chill for 2-4 hours, stirring occasionally.