How to give input for satellite image classification in matlab?

i m classifying a satelliite image on the basis of its NDVI values and i m classifying it into three classes. so plz tell me should i use numbers 1,2,3 for different classes in training data and will there be three columns in training data set? As i m classifying it into three classes so will there be three columns as well in input matrix? really need ur help...