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First off, let's assume this is a free-for-all melee, all 3 heroes have no talents, max skill levels, and max hero levels:
Moltanica, Arctica, and Demogorgon enter the Thunderdome at the same time. It only takes a second for Moltanica and Arctica to realize that neither of them can handle Demogorgon alone.
Moltanica and Arctica take the initiative, and begin double-teaming Demogorgon. Unfortunately for them, all this does is fill up Demogorgon's energy bar. Demogorgon inevitably procs first, an both Moltanica and Arctica suffer a horrible meaningless death.
The spectators leave somewhat upset, comparing the match to Ronda Rousey's recent losses. The result isn't a surprise though. A showdown between Demogorgon, Arctica, and Moltanica isn't much of a showdown. Kinda like watching an adult beat up children.