Toss the tomatoes, onions, and garlic with 2 teaspoons of the olive oil. Brush the salmon with the remaining teaspoon of olive oil. Arrange the tomato mixture around the salmon on a baking sheet. Broil 10 to 15 minutes, turning the tomato mixture at least once during cooking. Serve with the mixture on top of the salmon and finish with a dusting of salt, pepper, or seasoning of your choice.