Well, Mafia can't be outright
outed
, all you get is 1v1s when players counterclaim each other.
One obvious scum strategy is to each pick a distinct number in the 1-11 range (randomly but with no duplicates between scum), and act all game as though they had that number. Even if all three duplicate town numbers, scum still win 12.5% of the time (if they win all three of the resulting 1v1s; note that scum using this strategy will counterclaim a townie who claims their randomly picked number, so you can't use claim timing to distinguish scum from town in a counterclaim). Scum's victory chances are somewhat better if at least one of them happens to pick an unused number, thus becoming immune to being counterclaimed.
Some information for anyone wanting to do an EV calculation for strategies that work along these lines: there are 165 possible combinations of three numbers scum could pick.
1 of them has none of the scum counterclaimable.
24 of them have one of the scum counterclaimable.
84 of them have two of the scum counterclaimable.
56 of them have three of the scum counterclaimable.