There are a lot of trainers in Gothic 3 (because there is a lot of skills of course). I'm not going to list trainers teaching skills on first or second level - you can meet them almost in every town. I'm going to list only the masters (and not all of them - for example there are more than ten sword masters) and characters teaching you the rare skills.
Paladin - Rhobar (after choosing Innos).
Regeneration - Rhobar (after choosing Innos); Kan the leader of the Orcs in Faring
Master two weapon fighter - Ugrasal from Ishtar
Large weapon III - Hogar from the Wolf Clan (after duel); Ugolf from the Hammer Clan (after doing quest for him).
Orc Hunter - Hogar from the Wolf Clan (after duel).
Crossbow Master - Anog from Silden and Marik from Montera.
Bow master -Falk from Silden, Candela from Okara and Bogir from the Wolf Clan
Orc Hunter -Chris from Vengard outdoor, Armon from Cape Dun and Candela from Okar.
Dark Mage - Tizgar from Bakaresh.
Water Mage - Saturas from Mora Sul ruins, Vatras from Lago.
Mana Regeneration - Karrypto from Vengard, Saturas from Al Shedim, Sigmor from Bakaresh.
Learn quickly - Water Mages: Cronos, Merdarion, Saturas, Vatras, Riordian, and also Delazar from Ishtar.
Staff Fighting III - Ateras from Bakaresh, Shakyor from Lago.
Impossible pickpocketing - Peer from farm near Trelis.
Impossible Lockpicking - Lares from Geldern.
Knockdown - Lares from Geldern.
Murder - Ugrasal from Ishtar.
Prew Poison - Aldo from Bakaresh, Delazar from Ishtar, Eusebio from Ben Erai and Rasul from Lago.
Poison Weapons - Aldo, Delazar, Eusebio, Rasul and Esiel from Cape Dun.
Poison Arrows - Aldo, Delazar, Eusebio and Rasul.
Acrobatics -Pavel from Silden, Harek from Cape Dun (if you escorted him to Jens)
Endurance of the Wolf -Pavel from Silden and Torn the druid.
Weakness resistance - Renwik from Geldern.
Poison resistance - Peratur from Geldern.
Heat resistance - Sanuyem from Mora Sul ruins, Tufail from Braga.
Cold resistance - Bogir from the Wolf Clan, Lars from Silden (when you set him free).