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Based on the Turtlebot3 Burger core source for OpenCR in Arduino IDE, there are 5 publishers.
One of them is sensor_state_pub that publishes button status, voltage level, encoder values on "sensor_state" topic. (https://github.com/ROBOTIS-GIT/turtlebot3_msgs/blob/master/msg/SensorState.msg)
Similarly, imu_pub publishes IMU readings from OpenCR with "imu" topic.
You can subscribe these topics in order to receive data from OpenCR.

However, there is no subscriber in the OpenCR firmware nor publisher in RPi 3 Turtlebot3 source for LED control.
The latest OpenCR firmware already utilizes USER1~4 LEDs for TXD, RXD, Low Battery and ROS Connect indicator.
If you want to use these LEDs for other purpose from RPi3, you need to create publisher for LED topic in the RPi 3 and subscriber for the same in the OpenCR(with Arduino IDE) to control LED.

Based on the Turtlebot3 Burger core source for OpenCR in Arduino IDE, there are 5 publishers.
One of them is sensor_state_pub that publishes button status, voltage level, encoder values on "sensor_state" topic. (https://github.com/ROBOTIS-GIT/turtlebot3_msgs/blob/master/msg/SensorState.msg)
Similarly, imu_pub publishes IMU readings from OpenCR with "imu" topic.
You can subscribe these topics in order to receive data from OpenCR.

However, there is no subscriber in the OpenCR firmware nor publisher in RPi 3 Turtlebot3 source for LED control.
The latest OpenCR firmware already utilizes USER1~4 LEDs for TXD, RXD, Low Battery and ROS Connect indicator.
If you want to use these LEDs for other purpose from RPi3, you need to create publisher for LED topic in the RPi 3 and subscriber for the same in the OpenCR(with Arduino IDE) to control LED.

Based on the Turtlebot3 Burger core source for OpenCR in Arduino IDE, there are 5 publishers.
One of them is sensor_state_pub that publishes button status, voltage level, encoder values on "sensor_state" topic. (https://github.com/ROBOTIS-GIT/turtlebot3_msgs/blob/master/msg/SensorState.msg)
Similarly, imu_pub publishes IMU readings from OpenCR with "imu" topic.
You can subscribe these topics in order to receive data from OpenCR.

However, there is no subscriber in the OpenCR firmware nor publisher in RPi 3 Turtlebot3 source for LED control.
The latest OpenCR firmware already utilizes USER1~4 LEDs for TXD, RXD, Low Battery and ROS Connect indicator.
If you want to use these LEDs for other purpose from RPi3, you need to create publisher for LED topic in the RPi 3 and subscriber for the same in the OpenCR(with Arduino IDE) to control LED.

========Appended Answer========== Answer==========
I'm sorry for the unclear explanation about the OpenCR firmware. firmware.
In Arduino IDE, if you opened turtlebot3_core from "File > Examples > turtlebot3 > turtlebot3_burger > turtlebot3_core", the "turtlebot3_core" tab will be opened in default. default.
In the same file, go to line #313 where you can find "void publishSensorStateMsg(void)" function. function.
This function is called within the main loop (line # 203) of the code.

Based on the Turtlebot3 Burger core source for OpenCR in Arduino IDE, there are 5 publishers.
One of them is sensor_state_pub that publishes button status, voltage level, encoder values on "sensor_state" topic. (https://github.com/ROBOTIS-GIT/turtlebot3_msgs/blob/master/msg/SensorState.msg)
Similarly, imu_pub publishes IMU readings from OpenCR with "imu" topic.
You can subscribe these topics in order to receive data from OpenCR.

However, there is no subscriber in the OpenCR firmware nor publisher in RPi 3 Turtlebot3 source for LED control.
The latest OpenCR firmware already utilizes USER1~4 LEDs for TXD, RXD, Low Battery and ROS Connect indicator.
If you want to use these LEDs for other purpose from RPi3, you need to create publisher for LED topic in the RPi 3 and subscriber for the same in the OpenCR(with Arduino IDE) to control LED.

========Appended Answer==========
I'm sorry for the unclear explanation about the OpenCR firmware.
In Arduino IDE, if you opened turtlebot3_core from "File > Examples > turtlebot3 > turtlebot3_burger > turtlebot3_core", the "turtlebot3_core" tab will be opened in default.
In the same file, go to line #313 where you can find "void publishSensorStateMsg(void)" function.
This function is called within the main loop (line # 203) of the code.