Blend goat cheese, watercress, and truffle oil in processor until just combined. Season with salt and pepper. Place bread slices on work surface. Spread each with goat cheese mixture, dividing equally. Set aside 24 radish slices for garnish. Top 6 bread slices with cucumber slices, then remaining radish slices. Sprinkle with pepper. Top with remaining bread slices, cheese side down. (Can be made 2 hours ahead. Wrap individually in paper towels; chill.) Trim sandwich crusts. Cut each sandwich into 4 squares. Transfer to platter. Top each with 1 radish slice and serve.