I'm not 100% sure about how to get them, but I know a little bit about the events. Please note that you may not be able to get the items for them until the events are released.
Meloetta: If you don't already know, Meloetta has two forms. The move Ancient Voice is required if you want to change her forms. Do you remember the small cafe in Castelia City on Narrow Street? You need to go there with Meloetta. You should see a man playing a guitar right when you enter. Talk to him and he'll play a song on his guitar, triggering Meloetta to come out of her pokeball. Once she is done dancing, she will learn Ancient Voice, allowing her to change forms.
Voice Form: Normal/Psychic
Step Form: Normal/Fighting
Move Required: Ancient Voice
Kyurem: I actually have this pokemon on my team, so I know how to get it. To the right of Lacunosa Town, if you look on your map, you should see a spot called Giant Chasm. Go there and try to find a certain path leading to a small spot of water. This will trigger a scene where Kyurem screams and the entire place turns into a snow-covered blizzard. Walk around on the snow near the top until you see stairs. Go up the stairs into a cave and Kyurem should be waiting inside. He will be level 75, so be prepared.
Genosect: I'm not sure how to get him or Meloetta as I think they are both events that haven't been released yet. However, if you go to Route 18 (to the left of Nuvema Town) you should see ocean currents. Try to figure out which path you need to take until you see a warehouse sitting by itself on a small island. Inside, there should be a scientist working on something when he notices you. He will talk to you and challenge you to a battle. When you beat him, he should give you the Cassettes.
Keldeo: He is another event that hasn't been released yet. Once you have him, take him to the Moor of Iccirus. You also need to have the Musketeer Trio which is Cobalion, Virizion, and Terrakion. I'm not sure where you need to be, but you need to enter an open area at the Moor of Iccirus with Keldeo to trigger a scene where an old man walks up to you and talks. Make sure you have the Trio with you. They will also come out and will try to teach Keldeo Secret Sword. If you don't know where the Trio are at, I have them as well so don't worry. Cobalion is in Mistralton Cave, Terrakion is in Victory Road, and Virizion is in the Pinwheel Forest. Note that they will not show up until you defeat or capture Cobalion. If you accidentally kill Cobalion, don't worry. Leave for awhile, then come back. He should show back up. Same goes for Virizion and Terrakion. (I accidentally caused Cobalion to faint, lol). A fun fact, Keldeo is the fourth member of the Musketeer Trio when his home in the forest was burned down in a fire and the Trio rescued him. This was caused when humans and pokemon were at war long ago.
If you know when any of these events are, please tell me. I'm trying to get Meloetta and I hope this information was helpful!