The game Town of Salem is very similar to your idea.
Wiki link to the roles in that game
There are 3 main factions in this game, the Town, The Mafia, and Neutral. Generally, Only one of the 2 main factions (Town and Mafia) can win, while the Neutral have special win conditions. A lot of their Roles can be quite useful as general ideas, though there are too many to list out here, some good ones might be :
Investigator: Finds out clues about who others are
Sheriff: Can watch people
Spy: Spies on the Mafia.
Godfather: Can kill people
Consigliere (Consig): Finds out peoples roles
Consort: Distracts others, Preventing them from doing their roles.
Executioner: Trick Town to lynch an assigned target
Serial Killer: Can kill people
Edit: Your game is specifically 15 individuals, split 12 and 3, and 3 minimum deaths it seems. You would need at minimum 2 win conditions, One for each of these "factions". Even if the 12 have the same win condition, killing each other is completely possible if any of them have any reason to lie about their roles, and no way to prove someone is telling the truth. If the Architect has to write out their order in private, then someone could pretend to be the architect in public, while the real one just copies what they say to make them look the part. If the 12's goal is to have the 3 die, then they need to identify the 3, but the 3 would likely do everything in their power to hide the fact they are one of the 3. When 2 people claim to be a role only one person can have, you cant even be sure both are lying and a third person is the real one, hiding it for some other reason.
As well, Individuals an have their own win conditions. you seem to want that, but truthfully its not nessecary. The Jestor is the prime example though of how to use it. They win by getting targeted by the group as a whole to die. Meaning if the main goal of the 12 is to kill the 3, but one of the 12 is a Jestor, they might try to make you think they are one of the 3. By doing so, they get rescued and win, leaving 14 people, each now more likely to die because of it. If done to obviously however, they might just get secretly murdered and so not get rescued, helping the 3 win. The mere idea that someone is a Jestor means you cant truly rely on information not gathered by yourself, which is bound to cause deaths of the 12 due to simple mistrust. So long as the win condition for the 12 does not require 12 people, then confusion about who the 3 are will lead to deaths. If some of the individual win conditions include killing certain other individuals, possibly of the 12.