There are teams which had 11 players in squad but some of them have expired contracts.
The manager didn't log in to renew their contracts yet, so you see 5 or 6 of them in the field (and there are more but without new contracts)
The game gives few extra players 3* veterans so the team to have 14 eligible players for the new season.
If the manager is still absent and the team had 4 injuries (or 3 injuries and one red card) then doesn't have 11 players for the match so you have an auto-win with 3-0.
Unfortunately there is an issue with the game, that counts this win as a draw in your season's stats.
And there is another bad scenario, that tomorrow when that player with the red card will return, that team will have again 11 players and maybe in the next game your competitor in the League, might play and win him 10-0