HappyScans! & Gantz_Waitingroom release vol 3 with an "(end)".
But isnt it suppose to end at vol 5?
Or its 3?
Because after reading the v.3, I dont think its the ending yet.

If you look under 'Status in Country of Origin,' you can see three different compilation versions were released so what you've read was probably the Aizouban or Bunkoban version. But for better clarification, you should probably ask the Scanlator themselves.

After looking around in the scanlator blog, there's a hint that the Harlock manga didn't have a final conclusion.
And highly advise checking out the anime, released legally by discotek Media, for the conclusion of the war with the Mazone.

Yup, this gonna enter my "manga that has rushed/dropped ending" list.