Battleground : 3 same cards do not unify

1 card (Void Lord) is on the battleground, 2 others in my hand, but they do not merge / fuse / combine together.
I have a screenshot but cannot upload it :-/

https://us.forums.blizzard.com/en/hearthstone/t/how-to-share-links-to-screenshots-videos-and-replays/2242/6

were one the void lords a shifter by chance?