So yesterday when I called, they suggested I also call 800-ASK-USPS and report it to them so there would be a paper trail.
After being on hold with 800-ASK-USPS for 12 minutes, I told them the problem. I gave them my address, and the representative told me that they have this down as a single-family home, therefore we're only allowed to have one mailbox. I told them it's been a three-family home since 1893 when it was built. She told me that it's zoned as a one-family. I asked her where she's getting this information. She said it comes from the city. I told her that the city assessor's office has this down as a three-family home, we're taxed as three separate residences, the county has three deeds registered on the parcel, and so forth. She said that they're just going by the records they get from the city about how the home is "zoned," and I'd have to change "your zoning" with the city. Again, I explain that the city is well aware that it's a three-family home, and does she know what a zone is? So, like, can I send the USPS my tax bill or my deed or the printout from the assessor's office showing that it's a three-family home? Nope, they get their information right from the city, and I'd have to talk to the city about getting my home "zoned" as a three-family.
So, the end result is that she wouldn't take my complaint, because the USPS database says my address isn't allowed to have three mailboxes on it, so the carrier is actually delivering the mail correctly. "People think they can just cut a house up into apartments, but it doesn't work that way." Grr, my house has been a three-family home with three kitchens and three bathrooms since 1893. City records from 1893 onward show three unit numbers indicating the three different units.