I remember the minimum number of players you had to include on a match day roster for the match to be played normally was 3. They'd all play 48 minutes, together with some lucky fans etc. However I seem to recall a news post at some point about the game automatically filling incomplete lineups. I'm not sure if that referred to walkovers or all kinds of sub-5 player lineups and I haven't tried it since, so I'm not sure how it goes nowadays.
So I guess the question is simple: What will happen if I field 3 players for a scrimmage? Will they be the only ones who play, or will the game add the rest of my roster by itself?