I don't think the triple death was some kind of honour, I personally think the triple death was dealt someone who had committed terrible crimes, so even their soul would not be able to return. Tacitus describes this kind of death, as a punishment for incest and other unspecified crimes. We have bog bodies which resemble his description, so in this instance, I feel fairly secure in thinking Tacitus gave a true account. Then again, in the Irish Annals, there are accounts of one who commits crimes against the will of the gods as being forsaken to a triple death, which Fate then provides in a "Final Destination" perversity of circumstances. One fella, Muirchertach mac Erca, is wounded in battle so he is abed when a fire traps him in the house then finally drowns in a vat of wine when he tries to escape the flames, for instance.
Myrddin Wyllt phrophesied his own ending, killed by the triple death. The single death being one a magician could recover from. Celtic tales are littered with the theme enough for me to think it wasn't an honour and your soul didn't get to any afterlife, making it useless as a sacrifice, for the sacrificed couldnt take any messages to the gods.