The Witcher 3 hands you three skill trees and then quietly limits how many skills you can actually use. Slots, not points, are the constraint.
You unlock skill slots slowly, and an unslotted skill does nothing regardless of how many points are in it. So the build question is which twelve or so skills earn a slot, not which skills to buy.
Mutagens are the multiplier people miss
Skill slots come in groups, each attached to a mutagen slot. Fill a mutagen slot with a coloured mutagen and the skills grouped with it are boosted, but only if those skills share the mutagen's colour.
Red mutagens boost attack power and pair with combat skills. Green boost vitality and pair with alchemy. Blue boost sign intensity and pair with signs.
Matching four same-coloured skills to a same-coloured mutagen multiplies the bonus substantially. Mixing colours within a group throws most of it away, and this is the single most common reason a build feels weaker than the skill descriptions suggest.
The three builds, and what each is actually for
Signs. Blue mutagens, Igni and Quen. Igni burns through crowds, Quen makes you nearly unkillable. This is the easiest build in the game and it stays strong from start to finish.
Combat. Red mutagens, fast attack or strong attack specialisation. Highest raw damage, and it demands you actually learn the parry and dodge timings, since you are not hiding behind a shield.
Alchemy. Green mutagens, decoctions and potions. The most powerful build in the game at high level, and the slowest to come together. Decoctions grant enormous bonuses at the cost of toxicity, and the alchemy tree exists to let you carry more toxicity safely.
Euphoria, from the mutation system, scales damage with toxicity and turns the alchemy build into the strongest thing in the game by a wide margin.
Toxicity is a resource, not a warning
New players treat the toxicity bar as a danger meter and drink one potion at a time. Alchemy builds treat it as fuel.
With the right skills, sitting at high toxicity is where your bonuses live. Decoctions last for hours of game time and stack, so the routine before a difficult fight is to drink several and go in buffed rather than to save them.
Potions refill for free when you meditate with alcohol in your inventory. There is no reason to ration them.
Oils are free damage and almost nobody uses them
Applying the correct oil to your blade adds a large percentage of damage against that monster type, and after the relevant skill it never runs out.
This is the cheapest damage increase in the game and it requires nothing but reading the bestiary entry before the fight. A build with average gear and the right oil outdamages a better-equipped Geralt without one.
Gear sets, briefly
The witcher sets each support a playstyle: Feline for fast attacks, Ursine for strong attacks and vitality, Griffin for signs, Manticore for alchemy.
Upgrade one set through its tiers rather than wearing the highest-level mixed pieces you find. The set bonuses are worth more than a slightly higher armour rating, and diagrams for the next tier appear as you level.
The Witcher 3 walkthrough covers the main quest and the contracts worth doing.
