Go to Coinbase Pro
Log in or create a new account on https://pro.coinbase.com/
Open API page
Click on the profile icon in the top right corner of the page and choose API:

Click "+ New API Key"

Name your key
When you have multiple APIs, a proper name helps to manage it:

Select permissions and copy passphrase
For trading on 3Commas "View" and "Trade" permissions are required:

Note: please don't add addresses into the IP Whitelist field. Learn more here.
Enter 2FA code and click "ADD AN API KEY"

Copy API Secret
Save the API Secret key somewhere safe and click DONE:

You will see API Key created:

Connect Coinbase Pro exchange to 3Commas
Now you have all the needed information to connect your exchange with 3Commas.