Rental Insurance Question

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I am in the process of attempting to purchase my first multi-family dwelling.  It is a triplex.  When I called my insurance company, State Farm, to check out insurance rates so I can run the numbers, I was told anything over a duplex they consider commercial.  

They quoted me a duplex price, and a tri-plex price and it was almost an $80 difference per month for the exact same insurance rates.  Grrrrr....My first reaction was what a crock of crap, but my second reaction was to wonder if this is an industry norm.  Does every insurance company classify 3 and above as commercial or just State Farm?  

Where do you have your insurance through and what are their guidelines on this?  Also, what numbers do you use to know whether you are adequately covered, but not over covered?  

Thank you for your time and help!