Cut tomatoes in half; thread onto skewers, alternating colors. Brush with 1 tablespoon oil; sprinkle with salt and pepper.
Grill, covered with grill lid, over medium heat (300° to 350°) 10 minutes, turning skewers often.
Combine remaining 2 tablespoons oil, vinegar, and basil; drizzle over kabobs. Garnish, if desired.