You are going to need two scripts. One to process individual pressure plates, and one to detect finished condition. As for requiring a specific order, just put the pressure plate objects into the control array in the desired order with the editors drag and drop.The following is some untested pseudo code to get you going in the right direction.

PressurePlate.cs
////////////////////////////////////////////////////////////

using UnityEngine;
using System.Collections;

public class PressurePlate: MonoBehaviour {
   public bool isDown;

   void Start(){
      isDown = false;  // default to not down
   }

   void OnTriggerEnter( Collider other){
      // only set as down if a game object named "Player" has collided with me
      if(other.name == "Player") isDown = true;
   } 

}
/////////////////////////////////////////////////////////


PressurePlateCtrl.cs
/////////////////////////////////////////////////////////

using UnityEngine;
using System.Collections;

public class PressurePlateCtrl :MonoBehaviour {
   public PressurePlate[] myPlates;
   public Animation myFinal;    // door opening object animation

   void Start(){
     ResetPlates(); 
     myFinal.Stop();
   }

   void ResetPlates (){
      for(int itor =0; itor < myPlates.Length; itor++){
         myPlates[itor].isDown = false; 
      } 
   }

   void Update(){
      // dont process if myFinal animation has already run or is not attached
      if(myFinal == null) return;
      bool swapFlag = true; 

      for(int itor = 0; itor < < myPlates.Length; itor++){
         if(swapFlag){
            if(myPlates[itor].isDown == false) swapFlag = false;
         }else{
            if(myPlates[itor].isDown == true){
               Reset(); // true encountered after 1 false indicates out of order
            }
         }
      }
      // if swapFlag is true here, that means all plates are down in the correct order
      if(swapFlag == true){
         myFinal.Play();
     myFinal = null; // break connection so we dont reprocess
      }  
   }
}
///////////////////////////////////////////////////////