I tried implementing admob ads but didnt In app. Implementing ads worked for me without any issue. You can use Google test ads to test your project. To implement admob ads you need an admob account from there you will get your Publisher id and Android application ID. You have to put publisher id, Android application id and ad unit ID in construct 3
You can use the below link to get sample ad unit id for testing
developers.google.com/admob/android/test-ads
To get real ad unit id you have to create it in admob account.
There are lot of youtube tutorials regarding this. Still if you are in trouble dont hesitate to reply i can help you within my knowledge.Iam not a techie. (Sorry for my bad English)