1. Slice 2 of the bananas and toss with 3 tbs of the lime juice in a bowl. Add the yogurt and sprinkle with the cinnamon. Mix together; then process in a food blender or processor. Refrigerate covered for at least an hour.
2. To serve, divide the chilled soup between 4 ramekins. Thinly slice the third banana and toss it with the remaining tbs of lime juice. Lay the slices atop the soup for garnish. Serve chilled.