When I set some position inside walkable area, the character always ends in the middle of triangle. How can I change this to get him to exact location I want
You can edit the Seeker component -> Start End Modifier -> End Point. Set it to SnapToNode or Original.
Also adding a Funnel modifier to the gameobject usually helps a lot.
See this page for more information about navmeshes: http://arongranberg.com/astar/docs/getstarted2.php