If we take P&I and multiply by 12 payments then divide that number by our investment we will get yearly ROI?

Then the net should be divided between both partners.

A separate LLC, how ownership is divided, the LLC own checking account. 

@Kevin Martin Well, no, what the OP said was "ALL IN", this means that on the construction side, it includes the land, dividing/re-platting the lot(s), plans, permit, utilities, government fees, building construction costs, flatwork, landscape, and occupancy permit, on the contractor side, it shall also include any and all profit, overhead, and any other contingency, on the investor side, it also includes the research, buying and selling fees, holding costs, agent fees, and any other expenses associated with buying and selling.

Or anything to further that in the design (be it assigned parking, more private entrances, etc) could help.Or another (cheaper) angle is to somehow make the marketing scream triplex/divided home. 

Maybe this is too much technical, but seller needs to be prepared that the 25% taxes are not divided over the term of the entire installment term, rather than it is front loaded. 

The two car garage has a wall dividing it.

Make sure everything is spelled out clearly both gains and losses and how they are divided.

It's pretty easy to keep them divided without tons of extra work.

Keyword Density = Percentage of keywords you are trying to rank for divided by the overall word/content on the page.