First preheat oven to 375° F.
Begin by putting the butter in the microwave for 30 seconds to melt, then set aside to cool.
Next use a fork or potato masher to mash the three bananas, then use a large grater to grate the apples.
Mix the grated apple and melted butter into the banana.
Add the eggs, milk, and yogurt and mix well.
In a separate bowl (or just on top of the wet ingredients) mix together the dry ingredients then incorporate into the wet ones.
If adding chopped walnuts mix them in now.
Use a large ice-cream scoop or spoon to measure batter into greased muffin tins.
Bake at 375* for 15-18 minutes.