Is this really so difficult?
I’d start off agreeing with I, Brian. You would divy up the food available equally. You would equally share whatever sustenance is obtained from the island or the seas.
If that together is insufficient to sustain all three lives, eventually, one person dies. When that happens, they are placed on the menu.
You see, the OP introduces the fallacy that “There is only enough food for two of them.” How can that be? Whatever that supply is, wouldn’t the same amount be enough for one of them, for twice as long? That is, couldn’t you measure the amount of nourishment in “man-days”? If you had 60 man-days of food, that would be enough for two people for thirty days, three people for twenty days, or one person for 60 days. When that is exhausted, and one person dies, they simply add to the number of remaining man-days in the supply.
Is this really that difficult?