three files:
file A:
line 1: hello
line 2: good
...
file B:
line 1: morning
line 2: used
...
file C:
line 1: happy
line 2: sad
how to quickly save as a file with the following text
file A line 1:
file B line 1:
file C line 1:
file A line 2:
file B line 2:
file C line 2:
....
