I was following the instruction in this post

https://answers.ros.org/question/41433/multiple-robots-simulation-and-navigation/#261902

and I was able to launch 3 robots in the environment. the only problem is they can not use the map. I tested the map before using one robot and it worked.

does any one have an idea what could be the problem ??

the warning appears:

[ WARN] [1520822669.083386275, 5.051000000]: Request for map failed; trying again...
[ WARN] [1520822669.111106811, 5.078000000]: Timed out waiting for transform from robot2_tf/base_link to map to become available before running costmap, tf error: canTransform: target_frame map does not exist.. canTransform returned after 0.1 timeout was 0.1.
[ WARN] [1520822669.115123720, 5.082000000]: Timed out waiting for transform from robot1_tf/base_link to map to become available before running costmap, tf error: canTransform: target_frame map does not exist.. canTransform returned after 0.103 timeout was 0.1.
[ WARN] [1520822669.133585376, 5.100000000]: Timed out waiting for transform from robot3_tf/base_link to map to become available before running costmap, tf error: canTransform: target_frame map does not exist.. canTransform returned after 0.103 timeout was 0.1.

and these are the launch files I am using:

1st: one_robot.launch

<launch>
    <arg name="robot_name"/>
    <arg name="init_pose"/>
<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" 
    args="$(arg init_pose) -param /robot_description -urdf -model $(arg robot_name) " /> 
 <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/> 
</launch>

2nd: robots.launch

<launch>
  <!-- No namespace here as we will share this description. 
       Access with slash at the beginning -->
 <param name="robot_description" textfile="$(find robot_description)/urdf/myrobot.urdf" /> 

  <!-- BEGIN ROBOT 1-->
  <group ns="robot1">
    <param name="tf_prefix" value="robot1_tf" />
    <include file="$(find robot_description)/launch/one_robot.launch" >
      <arg name="init_pose" value="-x 0 -y 1 -z 0" />
      <arg name="robot_name"  value="Robot1" />
    </include>
  </group>

  <!-- BEGIN ROBOT 2-->
  <group ns="robot2">
    <param name="tf_prefix" value="robot2_tf" />
    <include file="$(find robot_description)/launch/one_robot.launch" >
      <arg name="init_pose" value="-x 0 -y -1 -z 0" />
      <arg name="robot_name"  value="Robot2" />
    </include>
  </group>

  <!-- BEGIN ROBOT 3-->
  <group ns="robot3">
    <param name="tf_prefix" value="robot3_tf" />
    <include file="$(find robot_description)/launch/one_robot.launch" >
      <arg name="init_pose" value="-x 0 -y -0 -z 0" />
      <arg name="robot_name"  value="Robot3" />
    </include>
  </group>

</launch>

3rd: move_base

<launch>    
  <!--- Run AMCL -->
  <include file="$(find amcl)/examples/amcl_diff.launch" />

  <!-- Define your move_base node -->
  <node pkg="move_base" type="move_base" respawn="false" name="move_base" output="screen">
    <rosparam file="$(find robot_description)/src/costmap_common_params.yaml" command="load" ns="global_costmap"/>
    <rosparam file="$(find robot_description)/src/costmap_common_params.yaml" command="load" ns="local_costmap" />
    <rosparam file="$(find robot_description)/src/local_costmap_params.yaml"  command="load" />
    <rosparam file="$(find robot_description)/src/global_costmap_params.yaml" command="load" />
    <rosparam file="$(find robot_description)/src/base_local_planner_params.yaml" command="load" />

  <remap from="map" to="map" />
  </node>
</launch>

4th: robots_nav.launch

<launch>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
 </include>

  <!-- start the world --> 
  <node name="spawn_model1" pkg="gazebo_ros" type="spawn_model"
    args="-sdf -file $(find robot_description)/urdf/wall.sdf -x 0 -y 0 -z 0 -model wall"/>   


  <!-- include our robots -->
  <include file="$(find robot_description)/launch/robots.launch"/>

  <!--Run the map server-->
  <node name="map_server" pkg="map_server" type="map_server" args="$(find robot_description)/maps/map.yaml" respawn="false" >
    <param name="frame_id" value="/map" />
  </node>

  <group ns="robot1">
    <param name="tf_prefix" value="robot1_tf" />
    <param name="amcl/initial_pose_x" value="0" />
    <param name="amcl/initial_pose_y" value="1" />
    <include file="$(find robot_description)/launch/move_base.launch" />
  </group>

  <group ns="robot2">
    <param name="tf_prefix" value="robot2_tf" />
    <param name="amcl/initial_pose_x" value="0" />
    <param name="amcl/initial_pose_y" value="-1" />
    <include file="$(find robot_description)/launch/move_base.launch" />
  </group>

  <group ns="robot3">
    <param name="tf_prefix" value="robot3_tf" />
    <param name="amcl/initial_pose_x" value="0" />
    <param name="amcl/initial_pose_y" value="0" />
    <include file="$(find robot_description)/launch/move_base.launch" />
  </group>

</launch>