house illegally transformed into apartment house

we purchased house converted into 3 plex 4 years ago come to find out it was done illegally in 1985 we are now told we have to convert it back to house. what do we do about this situation? any recourse. house will never rent for enough to pay the mortgage as a single home.

I don't have enough information to give you help. Your legal rights and recourses would depend on if the seller or realtor made any representations that the conversion was legal and how long your state gives a seller to sue. Talk to a real estate lawyer or your broker.