During charging, your Gear S3 and the charger may become hot. Exterior environment conditions (i.e. using for extended periods in very hot areas, storing device in very cold or very hot temperatures, exposure of device to direct sunlight. Etc) may also cause the device to heat up.
When the device heats up or rise to a specific temperature during charging or using, algorithm to protect the device’s overheating operates and "Battery needs to cool" message will pop up.
This is a normal operating condition to prevent damage to the device. Wait for the device to cool down before usage or charge the device again.