Options for 3 unit home zoned as a 2

[Ben Mitchell]

Hi all. I've noticed it's pretty common in the Boston area to see houses that have three apartments but are only zoned for two. Could anyone with experience in this situation explain some good ways to make use of that third apartment? 

For example, can the owner live in the third while renting out the first and second? Can the third be incorporated into the second and rented as one big unit? Can the third unit be used for STR?

I know that the ideal answer here is to get the zoning changed but in my experience that's rarely an option. Seems to me that in most cases, if it were possible, the previous owner already would have done it. 

Anyways thanks for any input.