Income Approach vs Past Sales

[Isaac Fridmann]

Hey BP Community,

First off, I just want to say how grateful I am to this platform. While I haven't yet acquired my first property, I'm way more knowledgeable thanks to the Bigger Pockets community and hope to also be able to provide value to others. 

I'm working hard to acquire my first BRRRR property. While I'm looking at SFH, I'm also focusing on duplexes, triplexes and quadplexes. I had 2 questions i was hoping to run by the community;

1) With duplexes, triplexes and quadplexes, do you calculate the ARV with the income approach or past sales?

2) When looking at comps for a triplex for example, if there aren't any that sold in the area, can you use a 3 bedroom SFH as a comp?

Thanks in advance!