From The Heroes of Might and Magic III wiki

Gold is the most used resource. Almost everything costs gold.

These things give gold:

• Town buildings
• Some Artifacts
• Some Heroes
  • Caitlin
  • Clavius
  • Nagash
  • Aine
  • Octavia
  • Damacon
  • Gelare
  • Grindan
• Secondary skill Estates
• Gold Mines
• Water Wheels
• Mystical Gardens
• Treasure Chests