I would say the third movie was bound to underperform regardless of rating or bootlegging. The second movie didn't do as well domestically as the first.
The gimmick of the original movie was all these action names being in the same movie. Then it turned out to be the Stallone and Statham show, then everyone else. Plus it never lived up to the hype. The second and third movies were Stallone front and center, then everyone else.
All three movies have been about CIA sending the Expendables in to take out a warlord with an endless army that shoots blanks, but every time the plot has been secondary to them just getting as many names as possible into one movie.