Peel and section grapefruit and oranges into a large bowl, adding juice as well.
Drain pineapples, reserving juice. Add pineapples to bowl with grapefruit and oranges.
Add 1 cup white granulated sugar and juice of two lemons and stir. (If you want to cut the sugar, I've used as little as 1/2 cup and it still tasted sweet.)
Dissolve gelatin in 1 cup of cold water.
Add water to pineapple juice to make 1 cup liquid total. Pour into a pan and bring to a boil. Combine hot pineapple juice with water and gelatin. Stir until combined and add to fruit.
If using a mold or small molds, line with maraschino cherries. Fill with fruit mixture and chill. If serving in a bowl or casserole, stir cherries into fruit and gelatin mixture and chill until set, usually 4-6 hours.