Answer:
(1) In this experiment, the variable that should be changed is the temperature at which the seeds are germinated.
(2) The variable that should be measured is the rate of germination, or the percentage of seeds that germinate within a certain time period.
(3) Some important variables that should be kept constant in this experiment include the type of seeds used, the amount of moisture the seeds are exposed to, and the amount of light the seeds receive. These variables can all affect the rate of germination, so it is important to keep them constant in order to accurately measure the effect of temperature on germination.