Co-Borrow an FHA loan on a triplex

[Thomas Ingrao]

A future partner and dear friend wants to house hack a triplex as his first home. He wants my involvement as a co-borrower and I think this would be a great opportunity to get involved on a deal at a very low entry cost. 3.5% down while rates are at 2.7% doesn't seem like I could go wrong.  

I would like some advice on how to structure the partnership equally, from beginning (down payment and closing costs) to end (cash flow). 

Here are some details on the market and a prospective home, for an example:

$124,000 asking price.

Two units are 1bed/1bath, other is 3bed/1bath. The rent market would bring around $1500 between the 3/1 and one of the 1/1 while he lives in the other 1/1. I am estimating a mortgage around $650-$800. I'm aware that there are plenty of cashflow calculators available, I'm just looking for advice on how to divide the cash flow after the mortgage is paid. Does anyone have experience with this type of situation? Thank you!