$ jruby --version
jruby 9.0.0.0.rc2 (2.2.2) 2015-07-09 ff331eb Java HotSpot(TM) Client VM 25.45-b02 on 1.8.0_45-b14 +jit [Windows 7-x86]

$ jruby -r open3 -d -e 'Open3.popen3("notepad")'
NoMethodError: undefined method `popen3' for IO:Class
  method_missing at org/jruby/RubyBasicObject.java:1597
          popen3 at C:/jruby-9.0.0.0.rc2/lib/ruby/stdlib/jruby/open3_windows.rb:74
           <top> at -e:1