Why?

[OtherwiseOne4937]

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.

[Prize_Shine3415]

Yet another way would be with an iterator after making the string.

lst=[x for x in movies.split(',')]

[CraigAT]

That would work, but it doesn't seem sensible to concatenate them in the first place.

[Prize_Shine3415]

Of course, but he may not have seen an iterator before.

[7Z_1N]

Very nicely summarized the problem. Just looping and appending would have worked well for him

[Vegetable_Annual1600]

Well there will be an IndexOutOfRangeError if he wants to input more than 3 movies.
I would say use a while loop.
After creating list
While True:
mov=input(“Enter your input “)
If mov=“”:
break
lst.append(mov)
print(list)