The gameplay is very similar to its predecessors in that the player controls a number of heroes that command an army of creatures inspired by myth and legend. The gameplay is divided into two parts, tactical overland exploration and a turn-based combat system. The player creates an army by spending resources at one of the eight town types in the game. The hero progresses in experience by engaging in combat with enemy heroes and monsters. The conditions for victory vary depending on the map, including conquest of all enemies and towns, collection of a certain amount of a resource, or finding the grail artifact. If a player loses all of their towns they will have seven game days to capture a new town. If they fail to do so they lose and the game ends. If a player loses all their heroes and towns, they will lose the game.
There are two "layers" to the world map: the aboveground and the underground. There are typically subterranean gateways that lead to and from the underground. Maps are filled with a huge variety of buildings, treasures, monsters, mines and so forth that reward extensive exploration. At the very least, a player must locate mines and flag them (whereupon they provide constant resources), since these resources are required to develop towns. The player must also develop their heroes' primary and secondary skills, both by battling creatures (and enemy heroes) and by acquiring artifacts or visiting special locations. Heroes are given a choice of skills to upgrade upon leveling up, as well as becoming better at combat or using magic. The skills must be chosen carefully, since they are permanent and only a limited number of skills can be learned.
The player's towns serve many functions, but most importantly they allow recruitment of creatures to form armies. Towns also provide funds, new spells and a fortified location to make a last stand against an invading enemy hero. To build new structures within a town requires gold and usually one or more type of resource. Wood and ore are needed for most structures, but more expensive buildings also require rarer resources (mercury, crystal, gems or sulfur). All factions require a disproportionate quantity of just one of these special resources, making the acquisition of a corresponding mine essential to victory. This same resource is also needed when hiring the most powerful creatures available to that faction. Each faction also has a handful of unique structures available only to them.
If a player finds the Grail artifact, they can deliver it to a town to make that town the Grail's permanent home by creating a special structure. The Grail bestows greatly increased creature growth and weekly income, in addition to a bonus unique to the town.
The eight different castles available in Heroes III are classified as good, evil, and neutral. Each town has seven basic creatures, each of which can be upgraded to a more powerful variant. Each town also features two associated hero types: one that leans more toward might (combat), and one that leans more toward magic.