This will largely depend on the city as some cities don't even require rental licenses but using for a non-permitted use is likely a misdemeanor. Zoning doesn't care about rental ordinances so you will have to talk to someone in building or in a smaller town maybe the city administrator who is familiar with city code.
If Mpls - you can live in one unit and rent the other two. From what I understand you currently can't use it as a triplex. You may be able to convert it to a triplex under the 2040 plan by adding fire barriers but a triplex is commercial building code. Duplex is residential building code so they are built to different standards. Maybe someone with more experience in Mpls will chime in.
First step here is to call the city and talk to them about it. It may have been a prior permitted use and is grandfathered in but you don't want to value a property as a triplex and be stuck converting to a duplex or having a vacant unit. There are many properties in Mpls that have this issue, it is up to the buyer to understand the permitted use for the property. Duplex with a non-conforming unit or ADU is not the same as a triplex.
Good luck.