Probably not anytime soon, depending on your definition of "comeback", though ruby is still chugging along just fine IMO. There are more IPO'd ruby-based companies out there now than there ever have been before. (edit: and many non-ipo successful exits where ruby usage is maintained)
> However, I see some attractive features in Ruby 3, do you think that's enough to get a comeback?
No, just a few nice additions to the language won't create any pop in popularity in any language.
> My main concern is jobs availability, as an employer I find it harder and harder to find good RoR developers and they become super expensive.
This is just a problem with tech jobs in general. I've had the same problem hiring JS devs, and they're the most plentiful. I also had the same problem 10 years ago staffing up RoR teams. Its not a new problem. It largely just depends on your definition of "good".