United, the three Deathly Hallows – the Elder Wand, the Resurrection Stone, and the Cloak of Invisibility – make their possessor the so-called Master of Death (this isn't child's play, folks). However, this isn't like an official title or anything; rather, it's a kind of symbolic way of saying that the successful possessor of all three Hallows is someone who has come to terms with death and therefore has attained a kind of mastery over it. As we see through the various histories surrounding the objects, the Hallows can't be used wrongfully to dodge or cheat death, like in the cases of the first two Peverell brothers. Instead, the Master of Death must come to understand the limitations and natural rules that apply to the powerful objects.
It's revealed that any desire for personal gain immediately cancels out the possibility of mastering one of the Hallows; when Dumbledore is tempted to use the Resurrection Stone to bring back his family and apologize to them, it fails. Only Harry, with his final acceptance of death, is able to unite the three Hallows for the first and last time, and become the Master of Death. He does so simply through his willingness to go with Death, without fighting or resisting, like his long-ago ancestor, Ignotus Peverell.
So, really, we see that the Hallows aren't actually about conquering death, or gaining power through magic, or anything like that – in the end, they're just symbols for the fact that death is an inevitable part of life, not to be feared or reviled.