Some investors I know figure out all material costs and triple it to include labor.

That would mean my last investment would have cost me around $24000 in closing costs, that would have been almost triple the actual cost.

I have done that model and seen that model over the years.. one guys in INdy called Holdfolio did thisnot sure how it performed but chose the rights assets and proper management and that works.there was a guy in Ohio also that called him self MR> triple net.. he sold you a home then rented it back from you for a set fee then he leased it out in a master lease situation were he took care of everything.my model I used a debt instrument equity share model that basically worked like what your talking about but had a pref return no matter what was happening with the asset.. that put big onus on operator to perform..

In the Cambridge-Somerville area, triple deckers in need of repair easily go for multi-millions, you need a lot of capital upfront for downpayment and rehab costs and you are relying on appreciation to build equity - the flip and condo conversion is a popular option in these areas. 

Looking at future growth rates, some estimates say that we should be able to double our population within the next 10-15 years and triple our population in the next 20-25 years.  

You'll be dealing with vomit, dirty toilets and sinks, clogged drains, drywall repair, double or triple occupancy in a single occupancy room, dealing with the people that are the causes of the aforementioned things, and probably complaints from neighbors.

Imagine locking in a 30 yr rate now, what will that comp rent amount be in 2030, double, triple?

One of the spaces my startup business will occupy and the other space is currently occupied by an existing business with a commercial triple net Lease in place.

As a beginner should I Invest in a double, triple, or quad?

In many states, you'd be able to sue for triple the deposit.