Rental Calculator use for short term, interest only, w/balloon ?

[Robert Steury]

The BB rental investment calculator seems set up for either interest only with no balloon or I&P with fixed term. I have a seller finance deal on the table where the seller is willing to carry an interest only note for 3 years with an I&P balloon at the end of 3 year term. I can't figure out how to make this work in the calculator. It seems set up for only fixed term I&P or interest only until paid off. Any ideas how to use the calculator creatively to overcome this seeming limitation? If not, suggest an outside source for making similar calculations with the financing structure I am trying to get my head around. Thanks all!