Househacking Taxes in South Carolina

I just bought a triplex in Greenville, SC and I am currently living in one of the units. My escrow was just raised by $300/mo because when I closed on the property, they assumed I was the primary resident and would be taxed at 4% of the $200,000 property. I just called the tax office and filled out the application for primary residence since they raise the taxes to the investment 6% rate. 

The tax office said they will only offer 4% on 1/3 of the property since I'm only a primary resident of one of the three units. I was under the assumption that they would tax the whole property at 4% since I was a primary resident of the whole. 

Does anyone have any advice to get around this?