Answer: The time that each type of ice cream is in the freezer must be the same in each experiment, because it can be a important factor in the experiment. Furthermore, they should repeat the experiment for various time lapses in the freezer, and check if in all cases the results of wich ice cream is the harder or softer remains the same.
Other thing they should do is check that the temperature of the 3 boxes of ice cream is the same before they put them in the freezer.