Here you find launch files to launch multiple turtlebot3 in Gazebo. It uses turtlebot3 packages to run. Turtlebot3 ROS documentation can be found here.
Install turtlebot3 dependencies as explained here and turtlebot3 simulation as explained here:
roscd
git clone https://github.com/cazevedo/battery_mockup.git
git clone https://github.com/cazevedo/cpr_gazebo.git
git clone https://github.com/cazevedo/robot_state_publisher2.git
git clone https://github.com/cazevedo/multi_turtlebot3.git
catkin build
Terminal #1
roslaunch multi_turtlebot3 home_world.launch
Terminal #2
roslaunch multi_turtlebot3 home_world_robots.launch
Terminal #3
roslaunch multi_turtlebot3 turtlebot3_navigation.launch