Halve two of the grapefruits. With a sharp knife loosen outside edge and between segments. Peel remaining grapefruit; section grapefruit over a small bowl to catch the juice. Set grapefruit sections aside. Measure juice in bowl. If necessary, add enough additional grapefruit juice or orange juice to equal 1/4 cup.
For dressing, add oil, mint, pepper, and salt to the 1/4 cup grapefruit juice in bowl. Whisk to combine.
Place one grapefruit half in each of four bowls. Top with watercress, avocado, and grapefruit sections. Drizzle with dressing.