Answer:
(1) The variable that should be changed is the temperature.
(2) The variable that should be measured is the germination rate of the seeds.
(3) The 3 important variables that should be kept constant are the type of seeds, the amount of water, and the amount of light. In order to isolate the effect of temperature on seed germination, it is important to keep these other variables constant so that they do not affect the results of the experiment.