how to interpolate between 3 position in a infinite runner game (scripts)

This script dosen't work !!!

 #pragma strict
 var speed=5.0;
 var speedtrans=50.0;
 var poscentre:GameObject;
 var posgauche:GameObject;
 var posdroite:GameObject;
 var centre:boolean;
 var droite:boolean;
 var gauche:boolean;
 
 function Start () {
 
 centre=true;
 droite=false;
 gauche=false;
 }
 
 function Update () {
 this.transform.Translate(speed*Time.deltaTime,0,0,Space.World);
 
         if(centre==true){
                 if(Input.GetKeyDown("a")){ transform.position=Vector3.Lerp(transform.position,posgauche.transform.position,0.37);
                          
                 gauche=true;
                 centre=false;
                 droite=false;
             
             }
             else if(Input.GetKeyDown("d")){ transform.position=Vector3.Lerp(transform.position,posdroite.transform.position,0.37);
                         
                         droite=true;
                         centre=false;
                         gauche=false;
                         }
         }
         else if(droite==true){
                     if(Input.GetKeyDown("a")){ transform.position=Vector3.Lerp(transform.position,poscentre.transform.position,0.37);
                         
                         droite=false;
                         centre=true;
                         gauche=false;
                     }
             }
                 else if(gauche==true){
                         if(Input.GetKeyDown("d")){ transform.position=Vector3.Lerp(transform.position,poscentre.transform.position,0.37);
                         droite=false;
                         centre=true;
                         gauche=false;    
                             
                             }
                 }