Auto name form and save to network

Any way to add custom file name to app.execMenuItem("SaveAs");?  When it saves, it does exactly what I'm needing - open windows explorer so users can drill down to the proper destination, which will be varied.  However, am having trouble finding how to name the file based on 3 pieces of form data. 

Example:

ContractNo_ProjectNo_ChangeNo.pdf

Any assistance appreciated, thank you!