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You are setting the robot_description parameter to the contents of your xacro file but it actually needs URDF data, which you use the xacro program to generate. The way this is normally done is something like:

<param name="robot_description" command="$(find xacro)/xacro '$(find three_wheel_robot)/urdf/robot.xacro'" />

Using the command attribute instead of textfile runs the value as a shell command and sets the parameter value to the output of that command.