It would be like this:
Black Kyurem: dragon/ice/electric
2x damage to dragon, fighting, rock, ground
0,5x damage to water, grass, flying, steel
0,25x damage to electric
White Kyurem: dragon/ice/fire
4x damage to rock
2x damage to dragon, fighting, ground
0,5x damage to electric, ice, bug, steel
0,25x damage to fire, grass
That would be awesome to have triple type legendaries. It's possible because it's really hard to choose one of three types to be omitted - dragon typing is their main typing, ice typing is their part of Kyurem and electric/fire typing is their part of Zekrom/Reshiram, so I really can't choose which typing should be omitted. And if there would be final Kyurem form it should be dragon/electric/fire because Kyurem would fuse with both Zekrom and Reshiram and lose its ice completely.