Crunchy could describe nuts, crackers, granola cereal. (I like my cereal to be crunchy, not soggy.) It also describes the ground under certain conditions, like snowfall that has refrozen on top, or mud that has dried. (The snow crunches under their feet.)
Crispy usually describes fried food like potato chips, chicken that has been battered and fried. (These chips are so crispy and salty.)
Crisp describes an apple and also is used for cool weather. (Crisp apples are better than mealy apples. I went for a walk on a crisp winter morning.)