A work vehicle comprising: an engine; a hydraulic pump driven by the engine; a work implement driven by hydraulic fluid discharged from the hydraulic pump; a travel device driven by the engine; a power transmission device that transmits driving power from the engine to the travel device; and a control unit for controlling the power transmission device; wherein the power transmission device has: an input shaft; an output shaft; a gear mechanism that has a planetary gear mechanism and that transmits rotation of the input shaft to the output shaft; and a first motor connected to first rotating elements of the planetary gear mechanism; a second motor connected to second rotating elements of the planetary gear mechanism; a third motor for assisting the first motor and the second motor; and a motor switching mechanism that is able to selectively switch between a connected state in which the third motor is connected to the first motor or the second motor and a disconnected state in which the third motor is disconnected from both the first motor and the second motor; and, the power transmission device is configured to change the rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speeds of the first motor, the second motor, and the third motor, and the control unit has: a connection determining unit that determines whether assistance from the third motor is required or not; and a motor switch control unit for controlling the motor switching mechanism so that, when the connection determining unit determines that assistance from the third motor is required, the third motor is set to the connected state, and when the connection determining unit determines that assistance from the third motor is not required, the third motor is set to the disconnected state. an engine; a hydraulic pump driven by the engine; a work implement driven by hydraulic fluid discharged from the hydraulic pump; a travel device driven by the engine; a power transmission device that transmits driving power from the engine to the travel device; and a control unit for controlling the power transmission device; wherein the power transmission device has: an input shaft; an output shaft; a gear mechanism that has a planetary gear mechanism and that transmits rotation of the input shaft to the output shaft; and a first motor connected to first rotating elements of the planetary gear mechanism; a second motor connected to second rotating elements of the planetary gear mechanism; a third motor for assisting the first motor and the second motor; and a motor switching mechanism that is able to selectively switch between a connected state in which the third motor is connected to the first motor or the second motor and a disconnected state in which the third motor is disconnected from both the first motor and the second motor; and, the power transmission device is configured to change the rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speeds of the first motor, the second motor, and the third motor, and the control unit has: an input shaft; an output shaft; a gear mechanism that has a planetary gear mechanism and that transmits rotation of the input shaft to the output shaft; and a first motor connected to first rotating elements of the planetary gear mechanism; a second motor connected to second rotating elements of the planetary gear mechanism; a third motor for assisting the first motor and the second motor; and a motor switching mechanism that is able to selectively switch between a connected state in which the third motor is connected to the first motor or the second motor and a disconnected state in which the third motor is disconnected from both the first motor and the second motor; and, the power transmission device is configured to change the rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speeds of the first motor, the second motor, and the third motor, and the control unit has: a connection determining unit that determines whether assistance from the third motor is required or not; and a motor switch control unit for controlling the motor switching mechanism so that, when the connection determining unit determines that assistance from the third motor is required, the third motor is set to the connected state, and when the connection determining unit determines that assistance from the third motor is not required, the third motor is set to the disconnected state.