Is there a way to do Admob ad mediation through Construct 3's Mobile Advert? ...
I think Admob Mediation only need configuration at your google admob account
You need to make Ad Units first (Banner/ Interstitial / Rewarded Video)
Then create a mediation and add that Ad Units
Make sure that AdUnits ID (which already added in Mediation) filled in your games event sheet in Construct3, depends on your events design.
So.. I think Construct 3 not directly related with "Mediation", it is just provide a space to display ads using ads id, and google admob will do the rest..
IMHO, CMIIW :)