Why does my home have three empty, inaccessible rooms?
Given the locations and sizes of those areas they are almost certainly pipe chases.  When drawing, framing, and drywalling a residence, it is common for spec builders to simply block out the space around pipes, supports, and ducts – with little effort to optimize the use or availability of such small spaces.

Of course, this means that there is a good chance you could use those spaces.  For example, when I run out of projects I will often cut an inspection panel into the drywall and, based on what I find:


Turned them into closets
Built-in bookshelves
Added discrete hatches for hidden storage