Getting the average order out of multiple lists

Question

Assuming that I have multiple ordered lists like this:

list_1 = ["apples", "oranges", "bananas"]
list_2 = ["apples", "pineapple", "grapes"]
list_3 = ["melons", "pineapples", "apples"]

How would I calculate the average order? I could loop over all items and get the average position for all lists. However what would I do when an item isn't in all lists? For example melons in the example above.

Thank you