global class BatchUpdate implements Database.Batchable<SObject>
    {
    global Database.QueryLocator start(Database.BatchableContext BC){

     return Database.getQueryLocator([SELECT ContactId,Contact.Email,Contact.HasOptedOutOfEmail,Contact.custom__c,OldValue,NewValue FROM ContactHistory WHERE Field = 'custom__c' AND CreatedDate >= N_DAYS_AGO:10]);
    }
          global void execute (Database.BatchableContext BC, List<Contact> scope){

    //Query to get all lists to map with contacts
            List<Lists__c> l1 = [Query List1];

            List<Lists__c> l2 = [Query List2];

            List<Lists__c> l3 = [Query List3];
    //

            List<Item__c> ilist1 = new List<Item__c>();
            List<Item__c> ilist2 = new List<Item__c>();
            List<Item__c> ilist3 = new List<Item__c>();

            Map<Id,Id> ctitem= new Map<Id,Id>();
            List<Id> pclist= new List<Id>();

    //Map each contact with associated lists

             for(ContactHistory ct: scope){
                 if((ct.NewValue =='Test1' && ct.OldValue =='Test')||
                    (ct.NewValue == 'Test2' && ct.OldValue =='Test')||
                    (ct.NewValue =='Test3' && ct.OldValue =='Test')){
                        for(Lists__c l : li){
                         ctitem.put(ct.ContactId,l.id);
                            ilisting.add(l.id);

                    }
             }

    // Query list of items to get if the combination of contact and List are already in the Item

            List<Item__C> ilistnn = [select id, Contact__c, Lists__c from Item__c where Contact__c =:ctitem.keyset()];
        }
           for(Id keyId : ctitem.keyset()){
            for(Id pf : ilisting){

    //Add the rest of the lists that are associated with the contact

                if(!ctitem.keyset.contains(pf)){

                    Item__C ic1 = new Item__c();
                    pc1.Contact__c = keyId;
                    pc1.Lists__c = pf;
                    pclist.add(pc1);
                }
            }
        global void finish(Database.BatchableContext BC){
        }

}