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Sumir SainiSumir Saini 

I can't add any custom components in exercise 3 of the Lightning App Builder module.

I enabled the checkbox in Lightning Component settings and still no custom components show up when I try to add them in exercise 3. 
SonamSonam (Salesforce Developers) 
Check if Lightning Components are enabled in your ORG..this happened to me as well when this feature wasn't activated in the org.