A camera switcher c# script for Unity3D created using version 2017, but should work with almost any of them.

VNCameraSwitch.cs

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

/// <summary>
/// A camera switcher. It cicles camera transform through given positions starting from a specified position walking 1 by 1.
/// It can either change or animate between positions with the specified speed.
/// </summary>
public class VNCameraSwitch : MonoBehaviour {

	// Store camera possible positions, including first position
	public Transform[] positions;
	// Current camera position, also used to set initial camera position
	public int currentPosition = 0;
	// The key that is going to change the camera
	public KeyCode switchKey = KeyCode.Alpha2;
	// The speed for animating between positions
	public float speed = 20f;
	// Choose between animate or not when changing positions
	public bool useAnimation = true;

	/// <summary>
	/// Sets the camera initial position to the specified currentPosition.
	/// </summary>
	private void SetCameraInitialPosition() {
		if (positions.Length > 0) {
			UpdateCameraPosition ();
		}
	}

	/// <summary>
	/// Check if the given position is the same as the camera transform.
	/// </summary>
	/// <returns><c>true</c>, if equals to camera position, <c>false</c> otherwise.</returns>
	/// <param name="positionToVerify">The transform with position and rotation to verify.</param>
	bool isEqualsToCameraPosition(Transform positionToVerify) {
		return transform.position == positionToVerify.position &&
			transform.rotation == positionToVerify.rotation;
	}

	/// <summary>
	/// Updates the camera transform to positions[currentPosition]
	/// </summary>
	void UpdateCameraPosition() {
		transform.rotation = positions[currentPosition].rotation;
		transform.position = positions[currentPosition].position;
	}

	/// <summary>
	/// Switchs the camera position when the switchKey is pressed and camera is right at currentPosition. 
	/// That means the camera is not allowed to change during animation.
	/// If useAnimation is true only the currentPosition variable is updated, changing camera's virtual position 
	/// to a desired position to be animated to.
	/// </summary>
	void switchCameraPositionIfNeeded() {
		if (Input.GetKeyUp (switchKey) && isEqualsToCameraPosition (positions [currentPosition])) {
			currentPosition++;
			currentPosition = currentPosition % positions.Length; 
			if (!useAnimation) {
				UpdateCameraPosition ();
			}
		}
	}

	/// <summary>
	/// Change camera transform towards desired position (currentPosition in positions)
	/// </summary>
	void IncrementPosition ()
	{
		float delta = speed * Time.deltaTime;
		Transform endPosition = positions [currentPosition];
		Vector3 nextPosition = Vector3.MoveTowards (transform.position, endPosition.position, delta);
		Quaternion nextRotation = Quaternion.RotateTowards(transform.rotation, endPosition.rotation, delta);

		transform.position = nextPosition;
		transform.rotation = nextRotation;
	}

	/// <summary>
	/// Animate camera towards new position with animation is on and is not between animations
	/// </summary>
	void AnimateIfNeeded() {
		bool shouldAnimate = useAnimation && !isEqualsToCameraPosition (positions [currentPosition]);
		if (shouldAnimate) {
			IncrementPosition ();
		}
	}
		
	void Start()
	{
		SetCameraInitialPosition ();
	}
		
	void Update()
	{		
		switchCameraPositionIfNeeded ();
			
		AnimateIfNeeded ();
	}
}