Go to Cex.io
Log in or create a new account on https://cex.io/.
Open account dashboard
Hover the profile ID on the top right corner, then click Profile.
Follow to API
You must enable:
Don't change other settings.
Note the secret key. You can't see it later.
Confirm with email
Open the email from Cex.io and confirm.
Add Cex.io to 3Commas
You can find it near the API key.