Is there any Booking/Reservation system for Craft 3?
Whilst one could build a booking system within Craft either with judicious use of Twig, Channels, and Entries or with a custom plugin it doesn't make a huge amount of sense to reinvent the wheel. Especially when booking management systems are deceptively complex when you get past the initial idea.

It would be possible to integrate pretty much any third party booking system into Craft with a bit of custom plugin development.