calculate months in which invoice will be generated

Guys,

How do I get my sheet to provide a cell that contains the correct months from the following info:

No of invoices ( this cell contains the number 1,2,3,4,6,12) and is OK

Month of 1st invoice ( this cell contains Jan, Feb, Mar etc ) and is OK

Invoices to be created ( this cell needs to read Mar, Jun, Sep, Dec, etc depending on how many invoices) and I'm stuck

Any ideas