Set arbitrary text to the left of each staff

Please see attached screenshot.
The bottom is a .pdf textbook excercise to fill missing tetrachords.
The top is the task completed in MuseScore.
But I need the top staff to be marked "а)",
the middle staff to be marked "б)",
and the bottom staff to be marked "в)",
as they are in the textbook.
(it would be "a)", "b)" and "c)" were it in English).
Currently all of them are marked "а)" - I renamed the instrument from "Piano" to "а)".
Renaming any of them renames all the rest.
Adding another instrument makes staffs connected with undesired vertical bars.
Just adding arbitrary text in an arbitrary position seems to be not allowed.
It feels it should be easy but I'm stuck.
Thanks