There are a lot of trainers in Gothic 3 (because there is a lot of skills of course). Im not going to list trainers teaching skills on first or second level you can meet them almost in every town. Im 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).