Smith -  It may come down to the occupancy level (owners vs renters) in your particular complex, and the state of the HOA (do they have healthy reserves).  

We haven't seen a drop in reservations with ours.

@Jeff PratherCodes would probably be the one that "gives you a hard time"To get it legally confirming to a triplex you'd have to go downtown and see what they say.

If I pay the 3.5 and saved the other 16.5% for reserves could I pay more down later?

So when I go and see the house I am checking for things that will confirm or change my mind. 

Three good steps are to 1) buy for cash flow, not appreciation, 2) secure debt that is longer in term than your projected hold period, 3) have adequate upfront reserves.

My initial reservation is that this estimate would more than double the current cash flow per unit, which seems like it might be difficult to do in practice - I'm not saying it's not possible, I am just hesitant.

I just need some direction on how I confirm the septic system is adequate for both structures.

I further agree setting up the reserve account is great. 

I just knew from experience how the repairs costs where going to go monthly and early, so I just put a reserve into a savings account for it.