Some options I've considered are finding houses with convertible space to add square footage but I worry that the cities will just not permit those conversions.

Depends what it's for - if we're adding square footage or something that needs to go on the deed then yes we'll do permits and an inspection Generally, if I know the contractor/sub does good work, I don't worry about a permit and inspection.

By increasing usable square footage, I was able to re-appraise and increase the value of my home by an additional 460 square feet of "livable space" and force the appreciation of my property's value.

Let’s assume the 4 bedrooms are 25% of the square footage of your home.Let’s say you make $80,000/year and you are in the 22% tax bracket.This will reduce the amount you have to pay on taxes over five years by: (Mortgage interest) x (tax bracket) x (the percentage of home used as rental) = $137,497 * 0.22 * 0.25 = $30,249.34 in tax savings.https://www.irs.gov/publications/p936https://www.irs.gov/help/ita/can-i-deduct-my-mortgage-related-expenses

If its a big property, as in they have a large square footage of land and that is where the money is tied up, they can rent storage space, farming space, etc.  

First, you will have to determine the square footage to each unit in the triplex.

Big amount of square footage in a high cost area. 3/3 made it attractive as a starter house.

As others have said, just go ahead and meet the minimum sq footages and try and not violate the other restrictions so that no one has cause to come after you for any violations.

From the outside they all look the same, but different square footage of cause.

rent by the room, rent by the unit (small MF), rent out an ADU, etc.If you plan on renting by the room, then maybe more square footage and potential rooms to rent.If it is small multi-family, then you can look at similar size rentals to see what their market rates are or use programs like Rentometer.com or BP rental rate calculator.