To connect or blend data in GDS you have to use the Blend Editor.
To access the Blend Editor you will need to add a chart or scorecard and select 1 data source.
Under the “Data Source” section you will see the blue option to “Blend Data”.
Click that and a new screen will open.
Now select “Join Another Table” and pick your next Google Ads account.
Once you do that click the button “Configure Join” and select “Inner”.
Then repeat those steps for the last 2 ad accounts and that should work!
Let me know if it doesn’t.