1 Message Text Thread Split into 3 Message Text Threads for Sibling Group

I belong to a large sibling messages text group. One member recently began using a different iPhone 13 along with an iPad. Now the single text thread has split into 3 text threads. Each single entry to the group goes to a different thread so we all have to read 3 different threads to follow 1 thought/conversation. I pinned the 3 groups at the top of messages screen just under the search bar so I can keep up with my siblings. Does anyone know how to merge these 3 text groups back into 1 text group?

Many Thanks!