In Hyperledger Fabric, network configuration is written into ledger (block). Therefore when you add a new organization to existing network (like org3), you have to create an “update transaction” and let orderer places it into a new block. Therefore there is a new block containing this transaction.

This is also exactly this tutorial is showing. If you know how to inspect the block (need some trick) then you can tell which block is doing what.

cheers, kc