failed to load ad : 3

W/Ads: Failed to load ad: 3

As per the Documentation you are getting the following error code:

public static final int ERROR_CODE_NO_FILL

The ad request was successful, but no ad was returned due to lack of ad inventory.

Constant Value: 3

Based on the post onFailedToReceiveAd - Ad request successful, but no ad returned due to lack of ad inventory when using admob with adwhirl:

If you are getting this error, then your code is correct. The issue is that AdMob does not always have an ad to return for every request. This may happen particularly if you have just registered your AdMob publisher ID, as it takes some time and multiple requests before the new ID starts returning ads.

Another reason your fill rate may be low is that you don't have AdSense backfill enabled, or you have filtered out some ads. Check your app settings to see if that is the case.

There is one option which helped in our case. As @blizzard mentioned in your application settings in Google Developer Console there is a section which called "Pricing and Distribution". In this section there is a checkbox "CONTAINS ADS". In our case it was disabled. After enabling we successfully received ads.

I had the same error in my app. I was launching the app in debug configuration. The problem was solved as soon as I run the release version of my app on the same device. In Android Studio just go to Build -> Generate Signed APK and choose the release configuration. Then install release .apk on your device. In debug configuration you can also check whether your test ads appears by adding AdRequest.Builder.addTestDevice("YOUR TEST DEVICE"). If it's ok with ads appearing, it means you just need release configuration.