I think you can only show Applovin ads in C3 via mediation. If you can't use Admob, the other two options are IronSource and Tapdaq.
IronSource addons are part of the Construct Collection, I think you can get it cheaper on itch.io, or chat with the developer at constructcollection.com , he might offer you a discount.
Tapdaq addons are only about $10, but I haven't used them and can't tell you anything about them.
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EDIT: Looks like there is also a Cordova plugin available. It may be possible to implement it without the C3 addon, but this will be a difficult task - you'll have to call JS methods and build with Cordova CLI.