I’m a Realtor and see conversions happen all the time. Typically, changing a single family into a multi-family residence can be difficult. However, if you are converting from a 2 to a 3 family, you’d have to apply for it at the buildings dept. They would assess if the third unit has the proper exits/entrances, ceiling height, etc. Historical districts add another layer of difficulty at times, but a lot of the historical homes in the area are converted to multi family dwellings these days so you should be OK.
Here’s a link to the Newburgh Buildings Dept
Hope that helps!
Keller Williams Realty