# Analysing the spread of data of a variable

In my data set there are repeated measures where each subject is measured at three time points.I want to see how the data of a single variable is spread.That is to find groups so that I can categorize the variables values into two or three groups. Why I want is after analyzing the data I want to use them to create membership functions for fuzzy logic. So for example if I have a variable as BMI and I want to categorize it as low, normal, high I want to find out 3 regions from the data set where I can group the data into the three groups low, normal, high.
What sort of method, graph can I use to analyze this repeated measures variable.