Preheat broiler to 400 F. Foil-line one baking sheet.
Brush bread lightly on both sides with 1 tablespoon olive oil; place on baking sheet. Broil about 2 minutes, turning once, until golden. Watch carefully to avoid burning. Remove from oven. Sprinkle cheese on one side of toasted bread; return to oven for about 20 seconds until cheese is melted and bubbly; cool completely. Cut bread into 1-inch pieces.
Combine tomatoes, cucumbers, onion, vinegar, basil and remaining 2 tablespoons olive oil in large serving bowl. Sprinkle with salt and pepper to taste. Top with bread pieces just prior to serving.