Could someone please explain how Attack and Defence affect Monster Stats and how Spellpower affects Spells?
Every creature has a base Attack / Defense score.
When combat starts, your creatures get a bonus to Attack / Defense equal to your Hero's Attack / Defense scores.
If a creature's combined attack is greater than their target's defense, they take an additional 5% damage per point of difference, up to a cap of +300% damage (when attack is 60 greater than defense)
Conversely, if a creature's combined attack is less than their target's defense, they deal 2.5% less damage, down to 70% (when defense is 12 greater than attack).
Spellpower's exact effect varies depending on the spell. For instance, Magic Arrow does (10 * spell power) + 10 damage.
Haste has a constant effect and lasts Spell Power turns in duration.
About defense reduction in accepted answer: 70% is delta (coefficient 0.3), so defense-attack limit is 28 (not 12). If hero has offense / armor - it becomes more complicated: coefficients 1.1/1.2/1.3 for offender-damage (direct melee, not retaliation) and 0.95/0.90/0.85 armourer-received-damage (for basic/advanced/expert level).
For precise formula one sould also count offense/armor specialists (crack,gundula / tazar,neela,mephala). Same for archery, ballista, spells, specialists for them (and for creatures: galthran/shakti etc.), creature capabilities (there is champion's bonus, but not vs alebards; angels produce 50% more damage against devils etc.).
Unfortunatly on mightandmagic.wikia.com formula is incorrect (e.g. attack/deffense should be separated), more correct manual on russian-funsite: section "combat", search by "D(bas)" (maybe google-traslate could give comprehensable result).
Since game is somewhat popular there is also: damage-calculator. One could always check VCMI sources, but i'm not sure if it's 100% correspondce to homm3 system.
For spell power, here's a list of spells and how Power affects them.
The number of turns these spells last in battle is equal to Power
- 🌪️Haste, Fortune, Precision, Air Shield, Protection From Air, Counterstrike, Magic Mirror
- 🔥Bloodlust, Curse, Protection From Fire, Blind, Misfortune, Fire Shield, Slayer
- ⛰️Shield, Slow, Stoneskin, Protection From Earth, Anti Magic, Sorrow
- 🌊Bless, Protection from Water, Weakness, Forgetfulness, Mirth, Prayer
These spells do damage based on Power with a multiplier ranging from 5 to 75. The additional numbers indicate the effect of Basic/Advanced/Expert Magic in that area (Air, Fire, Earth, or Water).
- 🌪️🔥⛰️🌊Magic Arrow (power * 10 + 10/20/30)
- 🌪️Lightning Bolt (power * 25 + 10/20/50)
- 🌪️Destroy Undead (power * 10 + 10/20/50)
- 🌪️Chain Lightning (initial hit power * 40 + 25/50/100)
- 🔥Fire Wall (power * 10 + 10/20/50)
- 🔥Fire Ball (power * 10 + 15/30/60)
- 🔥Land Mine (power * 10 + 25/50/100)
- 🔥Armageddon (power * 50 + 30/60/120)
- 🔥Inferno (power * 10 + 20/40/80)
- ⛰️Death Ripple (power * 5 + 10/20/30)
- ⛰️Meteor Shower (power * 25 + 25/50/100)
- ⛰️Implosion (power * 75 + 100/200/300)
- 🌊Ice Bolt (power * 20 + 10/20/30)
- 🌊Frost Ring (power * 10 + 15/30/60)
Health points affected
These spells use Power to determine how many health points of creatures are hypnotized, healed, or resurrected/reanimated.
- 🌪️Hypnotize (stack must have total health < single creature health * 25 + 10/20/50)
- 🌊Cure (heals power * 5 + 10/20/30)
- 🔥Sacrifice (resurrects health (power + destroyed troop's health + 3/6/10) * # sacrificed)
- ⛰️Animate Dead (reanimates health power * 50 + 30/60/160 [sic])
- ⛰️Resurrection (resurrects health power * 50 + 40/80/160)
Range on map
For this spell, Power determines how far away on the map from the character the spell can function.
- 🌪️🔥⛰️🌊Visions (power * 1/2/3, minimum 3)
The number of elementals summoned is based on Power and the level of magic (Basic/Advanced/Expert) for that type.
- 🌪️🔥⛰️🌊Summon Air/Fire/Earth/Water Elemental (summons power * 2/3/4)
Power is not used to compute the effect of these spells.
- 🌪️View Air, Disguise, Disrupting Ray, Dimension Door, Fly
- 🔥Berzerk, Frenzy
- ⛰️View Earth, Quicksand, Earthquake, Force Field, Town Portal
- 🌊Dispel, Summon Boat, Remove Obstacle, Scuttle Boat, Teleport, Clone, Water Walk
The source of this info is these pages:
I have just created damage simulator for calculation, enjoy: HOMM3 Damage Simulator