1. Place vanilla ice cream in refrigerator about 40 minutes to soften slightly. Meanwhile, finely chop 1/2 cup walnuts (reserve remaining 1/4 cup walnuts for garnish). In 9" by 3" springform pan, mix chopped walnuts, graham-cracker crumbs, 3 tablespoons butter or margarine, and 2 tablespoons sugar. With fingers, press mixture onto bottom of pan.
2. Evenly spread vanilla ice cream on top of crust; freeze until firm, about 45 minutes.
3. While waiting for vanilla ice cream to harden, prepare fudge sauce: In 2-quart saucepan over medium heat, cook cocoa, 3/4 cup sugar, 1/2 cup heavy cream, and 4 tablespoons butter or margarine {1/2 stick), stirring constantly, until mixture is smooth and boils. Remove saucepan from heat; stir in vanilla extract. Cool fudge sauce to room temperature.
4. Split 3 bananas lengthwise in half. Remove springform pan from freezer. Pour fudge sauce over vanilla ice cream; top with split bananas. Return cake to freezer; freeze 1 hour or until fudge sauce is firm. Place chocolate ice cream in refrigerator to soften slightly.
5. Spread chocolate ice cream over fudge and bananas. Return cake to freezer; freeze about 20 minutes to harden slightly. Place strawberry ice cream in medium-sized bowl; let stand at room temperature, stirring occasionally, until of smooth spreading consistency but not melted. Spread strawberry ice cream over chocolate ice cream. Cover cake; return to freezer and freeze until firm, about 3 hours.