Unity3D - Mission System

Hello Unity Community :) There is anyone that can help me with a script to manage simple mission? For example: the player must pick up 3 boxes with some ammo and gun and have to put it in a specific place. For example

var ObjectToPickUp : int = 3; var Object : GameObject;

And after taking the object you have to put it in a specific zone, so with trigger. when you enter in the trigger, appear the next object to pick up... Like in dead trigger mission Thanks :)