If a creature with myriad attacks in a game with 4 players and there is a Silent Arbiter in play, which of the three players being attacked gets to block?

Each opponent will be able to block.

When determining the legality of the block, you don't take blocking creatures controlled by other players into account. Since all of the blockers are declared at the same time, each opponent will be able to block the creature that's attacking them, even if other opponents also plan on blocking.

Nathan Long
Magic Rules NetRep