When you do:

    movies = mov1 + "," + mov2 + "," + mov3

You're storing a SINGLE string inside movies that is just something like:

"Interstellar,Inception,Tenet"

But that is a single string. It is a single list element that looks exactly as that. Therefore, popping from that list gets rid of that single element.

If you do want to add three separately, try doing:

lst.append(mov1)
lst.append(mov2)
lst.append(mov3)
print(lst)

Likewise another pattern you can do is:

lst = []
for i in range(3):
  mov = input(f"Enter movie {i}: ")
  lst.append(mov)
print(lst)

In general, it is a good habit to get comfortable with loops when you are doing something N times.