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:
roscdgit clone https://github.com/cazevedo/battery_mockup.gitgit clone https://github.com/cazevedo/cpr_gazebo.gitgit clone https://github.com/cazevedo/robot_state_publisher2.gitgit clone https://github.com/cazevedo/multi_turtlebot3.gitcatkin buildTerminal #1
roslaunch multi_turtlebot3 home_world.launchTerminal #2
roslaunch multi_turtlebot3 home_world_robots.launchTerminal #3
roslaunch multi_turtlebot3 turtlebot3_navigation.launch