i Everyone i am new in Salesforce 
So Please Help Me Out From this Scenario 

I have a two Custom Field on a Custom Object
1.ZipCode(Text)   

   Api Name=Zip_Code__c

2.Owner(Lookup User)
 

  Api Name=Owner

On Territory(Territory__c) Custom Object


My Condition is When i Insert a Zip Code like example:
1.332713=Aaron(Owner)
2.332713=Blake(Owner)
3.332713=Denise(Owner)

When i again insert 332713 and assign to a new Owner then Display a error we can not Assign a zipcode to this user Only a single zipcode is assigned to only three Owners(Sales Representatives)
Three Sales Representatives at most can be assigned to a single zipcode.Display an error if a user attempts to associate another sales representative to a zipcode

Please Replay As Soon Possible I dont know how to over come from this scenario so Please help me 

Thanks 
Neeraj Sharma