Undecided questions
I have a friend who is short and another one who is tall, if I have a friend who is in the middle?

You can say 'he/she is neither short or tall'. 'They are of average height,neither short or tall.

I have a friend who is short, another who is tall, and one that falls (or that is) somewhere in between.

"In the middle" is too specific. If you have one friend who is 1.60 and another who is 2 meters, the one "in the middle" must be around 1.80, no more no less.

If I had to use a single word to describe it, I'd either say that they are of "average height" or "middling height".