Eli Zaretskii writes:

>> Date: Sat, 15 Aug 2020 01:50:48 +0200
>> From: rgrant@posteo.net
>> 
>> Is this supposed to scroll down by three lines, or is it supposed to 
>> scroll down by three screen-fulls?
>
> The former.  The doc string says it clearly:
>
>   Scroll text of selected window upward ARG lines

The doc string is indeed clear, but I personally wouldn't have expected
that behaviour.  I'm curious if there's a particular reason why C-v
deviates from the convention of other similar movement commands, where
specifying a numerical ARG `n' would have the effect of performing that
command's action `n' times (e.g. M-3 M-f to move forward three words).