From The Heroes of Might and Magic III wiki
Artifacts are items founded in the adventure map. They can be picked up by heroes and equipped on them or put in the backpack, which can hold up to 64 unequipped artifacts. Hero can not pick up artifacts, if his backpack is already full.
Typically artifacts give the hero bonuses to his primary skills, enhance secondary skills or otherwise affect combat factors. Furthermore, they can give immunity to certain spells or increase the hero's movement on the adventure map.
 Artifact slots
Equipped artifacts are placed on squares on hero menu called slots as seen on the picture below. Slots represents different parts of the hero's body, and every slot can hold one artifact at the time.
|Slot 1 – Weapon: Typically gives bonus to attack skill.|
|Slot 2 – Head: Typically gives bonus to knowledge skill.|
|Slot 3 – Neck: Varies, but commonly gives spell immunities.|
|Slot 4 – Torso: Typically gives bonus to spell power.|
|Slot 5 – Ring: Varies.|
|Slot 6 – Shield: Typically gives bonus to defence skill.|
|Slot 7 – Shoulders: Varies.|
|Slot 8 – Feet: Typically gives benefits to hero's movement.|
|Slot 9 – Misc: Varies greatly. Note: In Shadow of Death and Heroes Chronicles there are five misc slots.|
|Slot A: Reserved only for ballista.|
|Slot B: Reserved only for ammo cart.|
|Slot C: Reserved only for first aid tent.|
|Slot D: Reserved only for catapult.|
|Slot E: Reserved only for spell book.|
|Slot F: Your backpack. Where you can store up to 64 unused artifacts.|