What’s the difference between stock Android, Android One, and Android Go? I'd like to know what's the difference between stock Android, Android One, and Android Go? 

Each is a flavor of Android, each originates from Google and all have a few things in common.

Is it just the Theme/Skin/UI that's different or is there something else?
Stock Android is meant for Pixel devices/other OEMs who like to keep the Android experience stock. Pixel devices are guaranteed to get 3 years of OS updates and security patches. Features, functions, skins, update policies all change when it is ported by other OEMs.
android-one is a Google initiative in collaboration with OEMs to provide stock Google experience to users of other OEMs while guaranteeing 2 years of OS updates and 3 years of security updates. See Two years of operating system upgrades (Android - one) - how is it implemented?. More importantly, users are expected to get quick updates but that's not been the case with all OEMs. Also, in theory it's supposed to be stock experience but OEMs don't always follow suit and cut corners.
Android-go 
is meant for entry level devices which have less hardware capabilities and less storage available (&lt;2 GB) . Goggle provides apps for these devices in Play Store. For example, Maps for Android go is smaller than the regular maps app and obviously has lesser features. For more see this. AFAIK, there are no guarantees for OS / security updates