It's because according to the
Versus Thread Rules only two participants are allowed in a handicap match (1v2), because any higher number of participants in a handicap can lead to misleading results, such as weaker participants relying on their stronger partners to win, making it too messy to properly structure on a character's Win/Loss section, as there'd be too many characters in each result.
Even if a 1v3 was allowed, Vader would stomp hard... Let me explain.
In Versus Threads we have a thing called
Standard Battle Assumptions, which is a list of assumptions applied to a thread if no alternate assumption is given by the OP (Such as location, Version of character, Distance they are from each other, etc), considering no specific key for the characters is given, it's assumed they're in their strongest state (Which means Legends Vader and Generation 2 Mjolnir Master Chief are being used in this battle), and considering no specific distance is stated, it's assumed Vader, MC, Arbiter, and Atriox all start at least 4 km away from each other.
Legends Vader is far superior in terms of Attack Potency and Durability because he is rated Planet level (5-B), The Arbiter and Atriox are only Building level (8-C), while MC is only Multi-CIty Block level with his strongest weapons (8-A), meaning Vader can literally obliterate them with any attack and he wouldn't be harmed in any way by the Halo team's hits. Vader can also paralyze and crush them with the force, not to mention he can kill them in a multitude of ways such as scrambling their brains, probing their minds and turning them against each other, or straight up incinerating their bodies from the inside, and Vader doesn't even have to move an inch as his force attacks can cover Stellar distances. It's no contest.
So here's a word of advice, the next time you make a Versus Thread, make sure to check the profiles of the characters you're using as well as the rules and guidelines I linked.