From The Heroes of Might and Magic III wiki

There are a few units that are immune to mind spells. This means that they are immune to the following spells:

• Blind
• Berserk
• Frenzy
• Hypnotize
• Forgetfulness
• Mirth
• Sorrow

[edit] Units immune to Mind Spells

• Giants/Titans
• Undead
• Elementals
• Golems
• Stone/Obsidian Gargoyles