Discard a card.
Each other player discards 4 cards.
The player who discarded a Treasure with the highest cost gains this from play, or if there was no highest, the player to your left gains it.
Each player (including you) draws 3 cards.
You discard 1 card and everyone else discards 4 cards. If a player has fewer than 4 cards in hand, they discard all the cards they have.
Pay attention to the costs of any Treasure cards in these discards, and put the highest costing one into the discard pile last to help keep track of it. If any player discarded a Treasure with a cost strictly higher than any other player's discarded Treasure (taking Potion costs into account, if playing with Alchemy), they gain this copy of Mercenaries from play into their discard pile. Highest cost refers to the cost to buy the Treasure card, not the worth it produces when buying with it.
If there is a tie for the highest costing Treasure, or it is unclear due to Potion costs, or no Treasures were discarded, the player to your left gains this.
If you have the highest costing Treasure, you gain the Mercenaries from yourself, putting them into your discard pile immediately.
Although the Mercenaries card leaves play right after being played, it still counts as you having played an Action for the purposes of cards like the Peddler, and you have still spent an Action to play it.
After Mercenaries leaves play, the last effect still resolves, and all players gain 3 cards.