Problem with getkey/getkeydown

I have this script, and it works, and I want the camera to be in Third-Person mode when starting, and then in a certain position (First-Person view) when I press "A" once, but currently it starts in Third-Person, and it only goes to First-Person mode when I keep "A" pressed. I already tried Input.GetKeyDown, and it doesn't work, the it just works when I spam-click "A", I know it has something to do with that it checks every frame or something. Please help me!

Script (please ignore commented lines and the code for zooming and stuff, I just want the transition to work, Thanks! xoxo):

 using UnityEngine;
 using System.Collections;
 
 public class TPCamera : MonoBehaviour
 {
     //CAMERAS
     public Camera MainCamera;
     public Transform StartPos;
     public Transform EndPos;
     Transform CurrentPos;
 
     //ZOOMING
     public int MaximumZoomIn = 20;
     public int MaximumZoomOut = 100;
     public int ZoomSpeed = 5;
 
     //PANNING
     public float turnSpeed = 4.0f;    
 
     //OTHER
     public float Smooth = 5f;
 
     //PRIVATE
     private Vector3 mouseOrigin;        
     private bool isRotating;    
     Transform standardPos;            
 
     void Start()
     {
         standardPos = GameObject.Find ("CamPos").transform;
         //Find the current position of the camera
         CurrentPos = MainCamera.transform;
     }
     
     void Update ()
     {
         //CAMERAS
         //Sets the Camerastate to the opposite of the current Camerastate
         /*if(Input.GetKeyDown(KeyCode.Tab))
         {
             Camerastate = !Camerastate;
         }
         if(Camerastate == true)
         {
             transform.position = Vector3.Slerp(transform.position, ThirdPerson.transform.position, time * Smooth);
             transform.rotation = Quaternion.Slerp(transform.rotation, ThirdPerson.transform.rotation, time * Smooth);
             ThirdPerson.enabled = true;
             FirstPerson.enabled = false;
             Debug.Log("Third");
         }
         else
         {
             transform.position = Vector3.Slerp(transform.position, FirstPerson.transform.position, time * Smooth);
             transform.rotation = Quaternion.Slerp(transform.rotation, FirstPerson.transform.rotation, time * Smooth);
             ThirdPerson.enabled = false;
             FirstPerson.enabled = true;
             Debug.Log("First");
         }
         //Sets camerastate to the Third-Person when true, and First-Person when false
 
         ////////////////////////////////////////////////////////////////////////////////////////////
 */
         
         if(Input.GetKey(KeyCode.A))
         {
             TransitionOne();
             //transform.position = Vector3.Slerp(CurrentPos.transform.position, EndPos.transform.position, Time.deltaTime * Smooth);
             //transform.rotation = Quaternion.Slerp(CurrentPos.transform.rotation, EndPos.transform.rotation, Time.deltaTime * Smooth);
         }
 
 
 
 
         else
         {
             TransitionTwo();
             //transform.position = Vector3.Slerp(CurrentPos.transform.position, StartPos.transform.position, Time.deltaTime * Smooth);
             //transform.rotation = Quaternion.Slerp(CurrentPos.transform.rotation, StartPos.transform.rotation, Time.deltaTime * Smooth);
             //ThirdPerson();
         }
 
     }
 
     void TransitionOne()
     {
         transform.position = Vector3.Slerp(CurrentPos.transform.position, EndPos.transform.position, Time.deltaTime * Smooth);
         transform.rotation = Quaternion.Slerp(CurrentPos.transform.rotation, EndPos.transform.rotation, Time.deltaTime * Smooth);
     }
 
     void TransitionTwo()
     {
         transform.position = Vector3.Slerp(CurrentPos.transform.position, StartPos.transform.position, Time.deltaTime * Smooth);
         transform.rotation = Quaternion.Slerp(CurrentPos.transform.rotation, StartPos.transform.rotation, Time.deltaTime * Smooth);
         ThirdPerson();
     }
 
         void ThirdPerson()
         {
 
             transform.position = Vector3.Lerp(transform.position, standardPos.position, Time.deltaTime * Smooth);    
             transform.forward = Vector3.Lerp(transform.forward, standardPos.forward, Time.deltaTime * Smooth);
 
         // Zooming Out
         if (Input.GetAxis("Mouse ScrollWheel") < 0) 
         {
             if (Camera.main.fieldOfView<=MaximumZoomOut)
                 Camera.main.fieldOfView +=ZoomSpeed;
 
             if (Camera.main.orthographicSize<=20)
                 Camera.main.orthographicSize +=0.5f;
         }
         
         // Zooming In
         if (Input.GetAxis("Mouse ScrollWheel") > 0) 
         {
             
             if (Camera.main.fieldOfView>MaximumZoomIn)
                 Camera.main.fieldOfView -=ZoomSpeed;
             
             if (Camera.main.orthographicSize>=1)
                 Camera.main.orthographicSize -=0.5f;
         }
         
         //Switch Perspective and Orthograpic
         if (Input.GetKeyUp(KeyCode.O)) //Change "O" to whatever you want to change the camera switch input key.
         {
             
             if (Camera.main.orthographic==true)
                 Camera.main.orthographic=false;
             
             else
                 Camera.main.orthographic=true;
             
         }
 
         if(Input.GetMouseButtonDown(0))
         {
             // Get mouse origin
             mouseOrigin = Input.mousePosition;
             isRotating = true;
 
         }
 
 
         
         // Disable movements on button release
         if (!Input.GetMouseButton(0)) isRotating=false;
         
         // Rotate camera along X and Y axis
         if (isRotating)
         {
             Vector3 pos = Camera.main.ScreenToViewportPoint(Input.mousePosition - mouseOrigin);
             
             transform.RotateAround(transform.position, transform.right, -pos.y * turnSpeed);
             transform.RotateAround(transform.position, Vector3.up, pos.x * turnSpeed);
         }
 
     }
 }