It really is no different than single family-detached homes.
Realtors include "below grade" for usable finished square footage if the basement is completed. Appraisers do not, and that is the main difference between us.
Above ground finished square footage does not change and universal. As far as calculating it? Whew that is a lengthy story, but for triplex's if you can find out the square footage for 1 unit, most likely at least 1 of the other units also will be that same size. A lot of measuring to find out the area. To find your answer more directly try calling an appraiser. Just google one.