This is not a legal single-family dwelling

It's a pre-1938 building that never took a CofO. Provided that the space is sufficient and egress is provided, you can use it for 1, 2, or 3 families. There's no law stating that a single family can't occupy multiple units in the same building.

True that there's no C/O and that the bldg was converted to house three families prior to 1929. There's also an Icard that confirms it's a three family. Last DOB job application (from 2014) confirms three families as well.

Icard: https://hpdonline.hpdnyc.org/HPDonline/PDFs/Icard_723526.pdf

DOB application: http://a810-bisweb.nyc.gov/bisweb/JobsQueryByNumberServlet?requestid=2&passjobnumber=320859941&passdocnumber=01

The scenario you provide, George - where three separate dwelling units are occupied by a single family - is perfectly legal, yes. But the floor plan (and the description) indicates that the building is currently configured as a two-family. You can't go around demolishing dwelling units just because you want to - especially without obtaining permits, especially in multiple dwellings and especially with all the rent regulation in place. There could be a tenant somewhere lurking in the wings.

Any potential buyer should ask seller to obtain either a C/O or a letter of new objection for the existing configuration. Wouldn't want any problems at this price.

Bio says it's currently configured for two families so some work must have been done without filing for permits to arrive at that current arrangement. It was registered as a 3 family as late as November 2020. and last DOB

Illegal renovations are very common in BK 1-3 family townhouses. Also, most buyers do not have much issue with I-card 3 family being used as 1 family with one kitchen. DOB does not seem to care either till you actually file for reno.

The trouble pops up when you want to sell.

^ Case in point.