You can have a common filter for all the reports on your dashboard.
For this, click on the edit button of the report that you want to filter. In the listener section, select the variable name of the filter. Click on Add and add the input parameter.
Ensure that the input parameter is added as a filter at the report level for all the reports that are listening to it.
Similarly, you can do this for all the reports on the dashboard.
Please refer the below blog for detailed instructions.
Please Note: You do not need to use iFrames for the reports unless necessary. Using iFrames will affect performance and each iframe will load individually.
Can you tell me for what use case you are using iframes?