1. It looks like you're using the rules for Contests with Multiple Participants on pg. 159, correct? Those rules (and the accompanying example) suggest two routes: each of the Hazekillers rolls separately and the best roll is taken or only one Hazekiller rolls, but gets a circumstance die (representing the help from the other two). As Narrator, I'd decide beforehand how much of a challenge this encounter is supposed to be. If it's supposed to be really difficult, I'd take the best roll. If I'm looking more for a quick cinematic butt-kicking from the Mistborn (such as when implementing the Epic Conflicts rule on pg. 191), I'd just use the circumstance advantage.
2. Similarly, if I'm using the Epic Conflicts rule, the Hazekiller's leather armor isn't going to matter; each point of damage will be dropping another of the Hazekillers. If I'm not, your Scenario B makes sense.
3. It will depend on how the Koloss is attacking or what it is defending against, but yes, they do get a considerable bonus. Koloss are extremely scary up close (like in the novels); make sure to keep some distance and use the terrain to your advantage. Running away is a perfectly acceptable strategy when facing Koloss.