I presume you have a table of the distances between each city, A to L
across and L to A down giving you a triangular grid.

You can use the MATCH( ) function to determine which column your
starting city is in and embed this within an INDEX( ) function to
obtain the mileage at the intersection of the row where your second
city is. The second city is then the starting city for the second
journey to the third city, so you can easily build up a composite
journey this way, and then just total the mileages.

You would need to specify the cities, and I suggested having validated
pull-downs using the list of cities you already have.

Hope this helps.

Pete