How to buy a property that's apart of 3 properties with 1 loan

[Will King]

I'm negotiating with a seller on a cash flowing duplex. He has 3 properties on one mortgage. He is selling the duplex. I tried to negotiate a subject to transaction before learning that he has 3 properties on one loan, which the seller says won't work because the duplex being one of three properties on one mortgage. I negotiated, we can put down 5% ($6k) of agreed price and pay him the first 6 months of PI payments at closing $7200). He accepted this offer. He is open to seller carry, what he want's a month is $1200. What other way could I possibly structure this deal? Our plan is to refinance in 6 months into a 20 or 30 year mortgage. This is my first time experiencing this situation. Has any one experienced this before? Thanks in advance for the feed back.