You would still have to manually integrate any Ads SDK that you'd want to use with AdMob mediation to your game?
Choosing your mediation networks
Google mediation supports over 30 third-party ad networks. Integrating mediation requires changes in these places:
1. Third-party ad network UI - Sign up for the third-party ad network and create a placement in their front end.
2. AdMob UI - Update your ad unit's mediation settings to include the ad network. It is also recommended that you set up ad network optimization for ad networks that support it.
3. App code - Update your app to include the third-party ad network's SDK, and an adapter library that communicates between Google and the third-party ad network to request and serve ads.
This plugin has support for more ad networks: https://www.construct.net/en/forum/construct-3/plugin-sdk-10/mobile-master-monetization-142097
You could also try Enhance, they support a lot of ad networks, altough it seems that the C3 runtime is not supported yet https://www.construct.net/en/forum/construct-3/plugin-sdk-10/enhance-sdk-integration-132463/page-2.