Im not sure what you mean. You CAN'T slot all 3 greater meridians. Only 1.
Can you come to the discord and share some screenshots?
Viewing post in Ascend from Nine Mountains comments
You need to pick only one to install big shard, all others must be installed with small ones. And each of three methods gives different stats of outcome, depends where you put the big shard, so pick one method you prefer, and install goldshard where it says. Also using all 3 slots with big shards did nothing special, you sill get same reward as if choosing only 1 of them, so no difference here after updates.
I can confirm the issue: when breaking through to Stage III, the three most powerful methods don't unlock. If I insert a Meridian Goldshard into the Great Meridian slot, this requirement simply doesn't count. I only use one goldshard, but it doesn't help. If it matters, the golem that drops this item was defeated in a team battle. I'm playing on version 0.6.17.