What could cause radon levels to be higher on the 3rd floor than the 1st and 2nd?
Those are some high numbers, 4-pico's is the standard "something needs to done" threshold. But, the difference in readings has to do with ventilation &amp; thereby natural convection.

All air will rise or be drawn-up in a 3-story building, drawing is what's happening with the 2nd floor's lower readings &amp; they're being added to the 3rd floor. Additionally, this rise or draw is magnified when exhaust fans are used or windows &amp; exterior doors are opened.

It's best to treat or remove Radon at it's before it enters the structure so the entire structure is then cleared or reduced of Radon levels. However, this could result in more tenant turnover &amp; higher rents...I'm just saying (I'm joking).