I think the square footage is way under quoted since the previous owners converted an attached garage to a master suite and built a mother-in-law suite in back of the detached garage. 

- Off square footage of the house alone?

@Richard Wilson as far as working to the numbers i just took duplexes within the same neighborhood, within 10% square footage, within a mile, that recently sold within the last 12 months and priced it out.

Square footage in listing was all lies.

I've thought about finishing a portion of the basement to add more square footage.

When the appraiser compared the property, he did so with other three unit SINGLE structure buildings with similar total square footage to the total of the THREE structures of mine combined.

For example: One of townhomes I am looking at had a similar size and sq footage sell for $350k a few months ago.

Should I be pulling comps on Single Family homes with similar total sq footage? 

They run about the same for every house (assuming square footage, bathroom count, etc are about the same)

A house directly behind us (more square footage, but also in original condition) sold for $390,000 seven years ago ($45,000 over list?)