World of Van Helsing: Deathtrap

There are a maximum of three stars available per level, per tier of the campaign. difficulty and performance affect stars earned.

easy is worth 1 star, normal is worth 2 and hard is worth three.

Difficulty availability per tier
Tier 1 - easy, normal and hard
Tier 2 - normal and hard
Tier 3 and 4 - Hard

Monsters making it to the gate or players dying will stop three stars being achieved.

Playing co-op or playing through the game on easy/normal to level first is the best way to obtain maximum stars. I had this by the end of tier 2.