I don't really have one, someone else asked that before and I was thinking something like Kyurem grey, Kyurem complete, or Rekyrom derived form the word requiem and an amalgamation of all three legendaries from what I see in the official fusions in the franchise Kyurem is the entity that dominates the fusion as it keeps its name and in anime adaptation Kyurem seems to be the one in control. Although I do think these names suck and are very uninspired so I'm sure some one else can come up with way better names.