Done

first.ru

@@ -0,0 +1,7 @@
+require 'rack'
+
+my_server = Proc.new do
+  [200, {'Content-Type' => 'text/html'}, ['<em>Hello</em>']]
+end
+
+run my_server

my_server.rb

@@ -0,0 +1,8 @@
+class MyServer
+  def call (env)
+    return [200, {'Content-Type' => 'text/html'}, pretty_response]
+  end
+    def pretty_response
+      (Time.now.to_i % 2).zero? ? ["<em>Hello</em>"] : ["<strong>Hello</strong>"]
+    end
+end

second.ru

@@ -0,0 +1,13 @@
+require 'rack'
+
+class MyServer
+  def call(env)
+    return [200, {'Content-Type' => 'text/html'}, pretty_response]
+  end
+
+  def pretty_response
+    (Time.now.to_i % 2).zero? ? ["<em>Hello</em>"] : ["<strong>Hello</strong>"]
+  end
+end
+
+run MyServer.new

third.ru

@@ -0,0 +1,2 @@
+require_relative './my_server'
+run MyServer.new