Three food items I cannot do without:
Peanut butter (peanut butter and banana sandwiches and peanut butter on toast)
Tuna (tuna and pickle sandwiches NOT tuna casseroles)
Cranberries (cranberry sauce makes a very good breakfast)
Potentially, I think if I was put on an isolated desert island, I could survive with these three things for a very long time...maybe with some bread and fresh fruit and milk thrown in there from time to time).