I'd introduce some class to manage output. It will also move the carriage 3 lines above to update the output every time (at the print method).

class StatusPrint
  def initialize
    @hash = {}
  end

  def declare_worker(name)
    @hash[name] = "Worker_#{name}"
  end

  def complete_worker(name)
    @hash[name] += "...done (#{name})"
  end

  def print(move_carriage: true)
    puts "\033[#{@hash.size + 1}A" if move_carriage
    puts @hash.values.join("\n")
  end

  def done!
    @done = true
  end

  def done?
    @done
  end
end

So your code will look like this

status_print = StatusPrint.new

workers = (1..3).map do |n|
  Thread.new do
    status_print.declare_worker(n)
    sleep rand
    status_print.complete_worker(n)
  end
end

print_loop = Thread.new do
  status_print.print(move_carriage: false)

  until status_print.done?
    sleep 0.1
    status_print.print
  end
end

workers.each &:join
status_print.done!
print_loop.join

puts '- ready -'

Also it won't work if you want to pipe your output so you'll have to add some command line option to disable updating the output and print this static text:

Worker_1...done (1)
Worker_2...done (2)
Worker_3...done (3)
- ready -

Also see https://blog.stevenocchipinti.com/2013/06/removing-previously-printed-lines.html/ for more about updating the output.

Eventually, I have created a gem to solve this problem:

ParaLines Ruby gem

Output sample:

Some minimal usage sample looks like this:

require 'paralines'

plines = ParaLines.new

plines.add_static_line 'Gems latest versions'
plines.add_empty_line

gems = %w[sinatra rspec paralines rails hanami]

plines.add_static_line '- Random order:'
threads = gems.map do |name|
    Thread.new do
        plines << name.ljust(15, '.')
        res = `gem list -r #{name} -e`
        plines << 'v' + res[/\((.+)\)/, 1]
    end
end
sleep 0.2
plines.add_empty_line
plines.add_static_line '- Sorted:'
threads += gems.sort.map do |name|
    sline = plines.add_shared_line name.ljust(15, '.')
    Thread.new do
        res = `gem list -r #{name} -e`
        sline << 'v' + res[/\((.+)\)/, 1]
    end
end

threads.each &:join

The source code for the detailed sample with output as on the GIF animation can be found on the gem page on the GitHub.