Hi Vicki
You might have some success with using the due date type "A specific day of next month". If you set it to 15, for example, no matter what date in February you use for the invoice date, the due date will be March 15. But it only works if the due date is in the next month, not three months away.
If it's more than a month away, you can try the due date type "Number of days from a specific day of month". If you pick 90 as the number of days and 15 as the day of month, then any invoice entered in February will have a due date of May 16.
Steve