They don't break down the fee % other than to say that "the higher the subtotal, the lower the percentage"
https://www.airbnb.com/help/article/104/what-are-guest-service-fees
The 3% charged to hosts is a transaction fee - I'm happy to pay that, as I don't have to worry about the guest showing up without a means to pay. I'd pay the same % to run a CC through Square.
Yes, it does add up - whether you see it as the guest paying 15%, or the host losing out on money that goes to a middleman, we've all agreed to pay a cut for the service Airbnb is providing. Airbnb connects buyers & sellers and the service fee pays the support staff that helps guests and hosts, goes to improving the web site/apps, and marketing for them (and us). If it seems to high, we can look for alternative spots to market our homes, but until someone offers a better service, it seems like a fair deal to me.