A simple camera orbit for mouse input.

CameraOrbit.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CameraOrbit : MonoBehaviour
{
    private Vector3 startMMBMousePos;
    private Vector3 startMMBTransformPos;

    private Vector3 startRMBMousePos;
    
    private Vector3 touchStart;
    public float groundZ = 0;

    private Vector3 currentZTouch;

    // Start is called before the first frame update
    void Start()
    {
        
    }

    private Vector3 camPos;
    // Update is called once per frame
    void Update()
    {
        // scroll
        Vector2 zoomSpeed = Input.mouseScrollDelta;
        this.transform.position = this.transform.position + (this.transform.forward * zoomSpeed.y * Time.deltaTime * 50);

        // pan
        if (Input.GetMouseButtonDown(2))
        {
            touchStart = GetWorldPosition(groundZ);
        }

        if (Input.GetMouseButton(2))
        {
            currentZTouch = GetWorldPosition(groundZ);
            Vector3 direction = touchStart - GetWorldPosition(groundZ);
            Camera.main.transform.position += direction;
        }

        // rotation
        if (Input.GetMouseButtonDown(1))
        {
            startRMBMousePos = Input.mousePosition;
        }
        
        if (Input.GetMouseButton(1))
        {
            Vector3 deltaPos = Input.mousePosition - startRMBMousePos;
            float yaw = (deltaPos.x / 2000) * 360;
            float pitch = (deltaPos.y / 2000) * 360;
            float roll = -pitch;
            
            this.transform.Rotate(Vector3.up, yaw);
            this.transform.Rotate(Vector3.left, pitch);
            this.transform.rotation = Quaternion.Euler(this.transform.rotation.eulerAngles.x, this.transform.rotation.eulerAngles.y, 0);
            
            startRMBMousePos = Input.mousePosition;
        }
    }
    
    private Vector3 GetWorldPosition(float z){
        Ray mousePos = Camera.main.ScreenPointToRay(Input.mousePosition);
        Plane ground = new Plane(this.transform.forward, this.transform.position + this.transform.forward * 10);
        float distance;
        ground.Raycast(mousePos, out distance);
        return mousePos.GetPoint(distance);
    }

    private void OnDrawGizmos()
    {
    }
}