How to add extra new term into multiple nodes
Install the Views Bulk Operations module.    
Create a view and add a filter to view nodes having term a and b.
add    a Bulk operations: Content field and in the field  settings under    SELECTED BULK OPERATIONS select  Modify entity values.
Save the view and from the Operations list, select Modify Entity
Values.    Choose the term field you want to add the terms to,
and then    select the terms you want to add
Select the Add new
value(s) to    FIELD, instead of overwriting the existing values
check box and Submit the form.