The three numbers are used to adjust the relative weight of each factor: temperature, rain, humidity. In the past users have reported that the default weights used by the Zimmerman method is too aggressive (e.g. the watering level reduces too dramatically with respect to temperature change). Therefore we provided the ability to change the weights. You can leave them as is (100%), but if you feel any of the factors should have less influence on the watering level, you can suitably reduce the weight.