how to trigger a Azure Synapse pipeline only when the other two Synapse pipelines have completed successfully

The most easy would be to add a copy activity at the end of each pipeline (the initial 2 pipelines) to upload a file in blob location. Create an storage event trigger on the 3rd pipeline which would trigger on arrival of files. Use get meta data activity to check for existence of 2 files in blob for the 3rd pipeline activities to proceed further else ignore the event trigger. The below blog : https://datasharkx.wordpress.com/2022/09/24/event-trigger-of-synapse-azure-data-factory-pipeline-on-arrival-of-nth-file-in-azure-blob-storage/ explains the end to end process