[Unity 3D] Testing camera pan,zoom, orbit for using mouse button. Setting camera rarget after you have attached this camera script to camera object. ・Hold left mouse button orbit ・Hold middle mouse button pan ・Hold ALT with right mouse button zoom

AimCamera.cs

using UnityEngine;
using System.Collections;

public class AimCamera : MonoBehaviour {
  public Transform target;
	Vector3 f0Dir= Vector3.zero;
	Vector3 upVal= Vector3.zero;
	float zoomDistance= 5;
	float theta= 0.0F;
	float fai= 0.0F;
	float loc_x= 0.0F;
	float loc_y= 0.0F;
	float panWeight= 0.5F;
	
	void Update() {
		if( Input.GetMouseButton(0) && !Input.GetKey("left alt" ) ) {
			f0Dir=  new Vector3(Input.GetAxis("Mouse X")*5.0F, -Input.GetAxis("Mouse Y")*5.0F, 0);
			if(	Input.GetKey("left alt")) {
				loc_x= -Input.GetAxis("Mouse X")*1;
				loc_y= -Input.GetAxis("Mouse Y")*1;
				f0Dir= Vector3.zero;
			}
		} else if( Input.GetMouseButton(1)&& Input.GetKey("left alt") ) {
			zoomDistance= zoomDistance+(-Input.GetAxis("Mouse X")+Input.GetAxis("Mouse Y"))*0.1F;
		} else if( Input.GetMouseButton(2) ) {
			loc_x= -Input.GetAxis("Mouse X")*panWeight;
			loc_y= -Input.GetAxis("Mouse Y")*panWeight;
		} else {
			f0Dir= Vector3.zero;
			loc_x= 0.0F;
			loc_y= 0.0F;
		}
		theta+= Mathf.Deg2Rad*f0Dir.x*1;
		fai+=   -Mathf.Deg2Rad*f0Dir.y*1;
		
		upVal.z= zoomDistance*Mathf.Cos(theta)*Mathf.Sin(fai+Mathf.PI/2);
		upVal.x= zoomDistance*Mathf.Sin(theta)*Mathf.Sin(fai+Mathf.PI/2);
		upVal.y= zoomDistance*Mathf.Cos(fai+Mathf.PI/2);

		transform.position= upVal;
		target.transform.Translate( Camera.main.transform.up*loc_y+Camera.main.transform.right*(loc_x), Space.World);
		transform.position+= target.position;
		Camera.main.transform.LookAt(target.position);
	}
}