Avocado, Corn, Tomato Salad

1 large or 2 small avocados (reserve a portion for dressing), diced
1/2 small red onion, diced
2 ripe tomatoes, seeded and diced
2 or 3 ears of corn, roasted and cut from the ear

Dressing:
Finely diced reserved avocado
1/4 c Olive oil
2 T Balsamic vinegar
2 T Lime juice
Salt and pepper to taste
1 t Italian herbs
1/8 t cumin