Three Kingdom is a combination of roguelike-inspired progression and the gameplay of a deck-building card game. At the start of a playthrough the player selects one of four predetermined characters,which sets a starting amount of health, gold, a starting relic that provides a unique ability for that character, and an initial deck of cards with basic attack and defense cards as well what potential color-coded cards tailored to that character they will see through the run. The goal is to work through several levels of a kingdom, each level having a number of potential encounters distributed in a branching structure with a boss character at the end of the level.
Encounters include monsters which vary in strength, campfires to heal or upgrade cards to more powerful versions, shopkeepers to buy cards, relics and potions as well as to remove cards from the deck, chests with random loot, and random choice-based encounters.
The deck-building game requires players to develop a strategy for their deck on the fly based on the cards they can obtain from loot and in synergy with the relics they obtain. Like other deck-building, too many weak cards or the unplayable cards dilute the player's deck, and the player must strategically decide which cards are the best fit to make the deck effective, when to turn down additions of cards, or when to spend money to remove cards as to maximize the deck's strength.
Combat is played in turns. Each turn the player receives a fresh hand of cards and three energy points. The player can play any combination of cards as long as they have the energy to pay for the energy cost of each card, and at the end of the turn, all cards not played are sent to a discard pile. Players' cards vary by the character but generally consist of attack cards to damage opponents, skill cards that buff themselves, debuff opponents, or add to their blocking power for the turn, and power cards where the effect remains in play until the end of combat.
Each opponent on the field will telegraph what move they will make: if they will attack and with how much damage, if they will block, cast a spell to buff themselves or debuff the player. In some cases, the opponents' attack or the player cards may add unplayable cards representing battle conditions like Wounds and Curses to the player's deck that can dilute a player's hand and may add negative effects while in the player's hand. The player can determine the best strategy to avoid taking any damage after they complete their turn. If the player's health drops to zero, the game is over and they must restart from the bottom of the spire. Otherwise, if the player defeats all the monsters in an encounter, they typically receive gold and a choice of one of three randomly-selected cards to add to their deck, if desired.
Three kingdom metagaming aspects. Completed or failed runs contribute points towards unlocking new characters or new relics and cards that will be made available for the specific character. Ascension mode provides several challenges to the player such as to climb the tower with a character using only their starting relic or without adding any rare cards to their decks. A daily challenge gives players a single chance to get as high in the spire as they can under pre-set conditions and a fixed random seed, so that each player is starting from the same point and see the same encounters.