Hi, I have added an in app purchase on my construct 3 game which I recently put on the Google Play store and it works perfectly so I might be able to help you with your project.
1) Double click on a layout and add the MobileIAP plugin
2) Double click on a layout and add the Google Play plugin
3) Click on the Google Play plugin and in the properties window and on the Android ID box, type in your Application ID (Android) (e.g. com.companyname.nameofyourapp)
4) System > On start of layout > MobileIAP > Add product ID "type whatever you want it to be" with type consumable or non-consumable
5) System > On start of layout > MobileIAP > Complete Product registration
6) GooglePlay > On signed in > GooglePlay request player details
7) GooglePlay > On signed out > GooglePlay sign in
8) Touch > (whatever object you want) > MobileIAP > purchase product ID "type in what you did on step 4"
9) MobileIAP > Has product "what you typed on step 4") > do what actions you want to do... (e.g. hide banner ads, create extra level, change some global variables etc)
Tip on step 4, make sure the product ID does NOT start with a capital letter or a number.
Then when you export your apk and upload it to the google play console, set up a merchant account if you have not done so already and you can scroll down to the monetize section on google play console, select in-app products and click to create one. Just remember to put in the exact product ID from step 4 and set a price.
Hope this has helps you and anyone else looking at this topic