At 3M+ Impressions per day what is your exact fill %? Also what is your breakdown of impressions by platform You can potentially look to other ad partners to fill your supply. You can either plug into other ad-exchanges, the inventory that is not getting filled. The business model is the same, you are selling ads on CPM basis, you essentially just need to plug in the best ad SDKs as possible to fill as much inventory as possible.

Google Admob has relatively decent targeting, however, there are other options to ensure you get a high fill %.

My advice would be to plug your ad inventory into the exchanges. (e.g. Mopub, etc)