Hm, funny that it ain't written in the rules but there's a plain mafia member role in MG, it has been in games multiple times.
In a 13 people game, 3 plain mafias seem a bit 'weak' tho (with 14 players, they'd have 4 members) so we should expect 'surprises'. They mentioned that secondary roles are hidden, so that could be another indication. On the other hand, town has mostly vanilla members, but still numbers wise I'd be expecting someone to turn sides cause that's what that rule is about, imo following weird interactions or attention seeking gestures could be worthy and I'm saying this from the start to maybe cause some anxiety on said person(s) if that's the case. If we ignore the secondary roles mention tho, then a vanilla being recruitable wouldn't fit what I just mentioned, so hopefully behavior shifts later on would be noticeable. But uh... don't expect those 3 hosts to add some "spice" that is easy to narrow down tho... But yeah, sharing my thoughts on the setup so far.