Peel and section the oranges and grapefuits, working over a bowl to catch juices; set juices aside. Reserve 24 orange sections; dice the remaining sections.
Slash rounded end of each avocado three times, cutting about three-quarters of the way to the opposite end each time. Turn flat sides up and insert an orange section in each slash.
Brush avocados with reserved fruit juices. Heap the diced oranges and grapefruit sections into avocados. Garnish with pomegranate seeds. Serve on lettuce with Celery Seed Dressing. (Refrigerate any remaining dressing for another use.) Makes 8 servings.
Celery Seed Dressing
In a small mixing bowl, blender container, or food processor bowl combine sugar, lemon juice, celery seed, dry mustard, paprika, and salt. With mixer, blender, or food processor running, slowly add salad oil in a thin, steady stream. (This should take 2 to 3 minutes.) Continue mixing, blending, or processing until mixture reaches desired consistency. Makes about 1 1/4 cups.