Middle of screen vector 3?

Use Camera.ViewportToWorldPoint.

function Update () {
    var cam = camera.main;
    var v = cam.ViewportToWorldPoint(Vector3(0.5, 0.5, cam.nearClipPlane)); 
    transform.position = v;
}

Attach this script to the object you want to have in the middle of the screen.

It is impossible for any specific 'number' in world space to correspond to the exact middle of the screen- it always depends on the size of the screen, the position of the camera, and the location of the object itself!

Besides that, because a screen transforms a three-dimensional space into two dimensions, any single point on the surface of the screen will naturally correspond to an infinite ray projecting out into your game world.

Which brings me to my next point- Camera components have this wonderful functionality called

 Camera.ViewportPointToRay(Vector3);

in which you give it a point on the screen, and it returns a ray shooting off into your world!

If you know how far away from the screen you want your object to be, you can find that position by using

 correctPos = Camera.main.ViewportPointToRay(new Vector3(0.5F, 0.5F, 0)).GetPoint(distance);