How to get (the vector3) of where the mouse left clicked on the terrain?

Questions & Answers
1

How to get (the vector3) of where the mouse left clicked on the terrain? Used for RTS (real time strategy)

2

Conceptually, the user by clicking has fired a ray into the scene. You want to see where it hits.

First you need to use the current camera to translate the clicked location on the screen into the ray: Camera.ScreenPointToRay

Now you need to detect where on the terrain the ray hits. Fortunately terrain comes with a collider. You just need to ask that collider where the ray hits it, if anywhere. You can use Collider.Raycast for this. It will return a RaycastHit structure which contains the collision point as well as other useful information.