I did take a quick look at the extra Rocket Booking and the functionality seems quite limited. From what I can tell, there is no way to book a seat from the frontend. I believe every booking has to be handled manually in the manager.
What are these “everything else” extras? Sometimes it doesn’t take much effort to make an extra run on MODX 3.
If you have some programming skills, then you could create a simple booking system yourself, for example with MIGXdb as described here:
One problem is though, that I don’t know if the MIGX “Package Manager” works yet in MODX 3. You probably have to generate the class files on a MODX 2.x installation and then move the package folder (in core/components/your-package-name) to MODX 3.