To start a Gazebo simulation with TurtleBot3, type
roslaunch turtlebot3_gazebo turtlebot3_empty_world.launch
for an empty world, or
roslaunch turtlebot3_gazebo turtlebot3_world.launch
for the ROS world. To control the robot with the keyboard use
roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch
To launch RVIZ as well, type
roslaunch turtlebot3_gazebo turtlebot3_gazebo_rviz.launch
