Stealth III is one of Evie's three exclusive skills and the biggest single Stealth boost in the game. It needs level 5 and Stealth II.
Stealth III sits at position 13 of 19 in the Stealth tree. Unlocking Stealth III takes 4 skill points. You need level 5 and Stealth II before you can buy it. Stealth III is one of Evie's three exclusive skills, so Jacob cannot take it.