Line a 9- x 5-inch loaf pan with plastic wrap, allowing the plastic wrap to extend over edges of pan. Spread strawberry sorbet evenly in pan, packing into corners and bottom. Top with an even layer of mango sorbet, then lemon sorbet. Cover tightly with plastic wrap, and freeze at least 4 hours or until very firm. (Freeze up to 3 weeks, if desired.)
To serve, remove pan from freezer; let stand for several minutes. Run a knife around edges of pan. Invert terrine onto cutting board or serving platter; remove pan and plastic wrap. Cut crosswise into 8 thick slices; place on chilled serving plates. Top with berries, chocolate sauce, or fruit, if desired.