About 1-1/2 pints raspberry sherbet, slightly softened
1/4 cup miniature semisweet chocolate chips
Line a 1-1/2-qt. round metal bowl with plastic wrap. Press a thin layer of lime sherbet against the bottom and sides of bowl. Freeze, uncovered, until firm. Spread a thin layer of pineapple sherbet evenly over the lime sherbet layer. Freeze until firm. Pack raspberry sherbet into center of sherbet-lined bowl. Smooth the top to resemble a cut watermelon. Cover and freeze until firm, about 8 hours.
Just before serving, remove bowl from the freezer and uncover. Invert onto a serving plate. Remove bowl and plastic wrap. Cut into wedges; press a few chocolate chips into the raspberry sherbet section of each wedge to resemble watermelon seeds.