Depending where it is, you might be in a world of hurt. It might be impossible for it ever to become a triplex.
I'm in the Greater Toronto Area and in this area, depending on what city you are in, the rules are different for duplexes and triplexes.
I was the proud manager of an illegal triplex, and when the basement tenant called the fire department after pulling out a smoke alarm battery, we ended up having 30 days to do the work.
1- First we had to evict all the tenants because of the amount of work required.
2- We had to rip off all the drywall on the stairwells and insulate the stairwells
3- We had to drywall with a 2nd sheet of drywall the ceiling of the main floor
4- We had to drywall the entire basement ceiling throughout.
Those were the main issues and the changes from duplex to triplex. The owner kept asking me if he could sell and didn't have the money for the work.
It was ZONED properly, if it wasn't, it just would never be permitted, and you overpaid for nothing because of a slick sales job.