Mahler’s Third, which clocked roughly an hour-and-three-quarters this evening and requires women’s chorus, children’s chorus, alto soloist, and orchestral heft including eight horns, is more seldom programmed than his Second, or Bruckner’s Seventh (a mere hour-and-ten-minutes). But, Nézet-Séguin commands this monumental epic assuredly, not wasting a drop of the legendary Philadelphia institution inherited from legendary maestros such as Stokowski and Ormandy. And Mahler’s Third is as Mahlerian as his symphonies come — a broad canvas on which he depicts and explores “nature,” and the human experience, in the broadest sense.