How is a triplex appraised?  

The reports are 40-90 pages long and go through the entire house in great detail.Get an appraisal.

The house next door could have sold under extenuating circumstances and if its way off the market an appraiser might not even use it as a comp.  

The reports are 40-90 pages long and go through the entire house in great detail.Get an appraisal.

Because you took out the loan as a Non-Owner and you don't have a tenant in it yet, the appraiser will say what the market rents on the property will be.

The duplex is currently rented out, so I will look to do my best to get the home inspection lined up at the same time as the appraisal, Do you feel the inspection is necessary?

Most of them do not flip these properties because after paying down payment, closing costs, attorney fee, appraisal/inspection, holding costs, agent commissions, and transfer taxes, most of these deals don't make financial sense unless they hold onto the property for at least 5-10 years.That being said, most of them have done this to house-hack or use the BRRRR Strategy. 

Their job is to bring you an opportunity, not be an appraiser, designer, contractor, inspector, therapist or fortune teller.

At 90 days of ownership, or when rehab is complete if longer than 90 days, you can refinance to 75% of the new appraised value into long term rental loan (6-12 months ownership if you want to refi w bank).

This differs from an investment mortgage where they'll count 75% of an appraised long-term rent on the short-term rental as income - even when you go to apply for another mortgage later.