I am trying to install various linux distros on my MacBook Air and MacBook Pro machines. They often require an ethernet connection for the installation. Instead of paying for and waiting for an adapter (because the Mac machines do not have ethernet ports), is there a way I could use my Pi in the place of an adapter? I have a Raspberry Pi 3 B+ btw.
No! The USB (which is a Hub) cannot act as a host. It may be possible with a Zero or A, but even this is impracticable.
But then your question is predicated on falsehood - you DO NOT require an ethernet connection for installation. You can easily install from an image on USB, but this is not a Pi question.