i assume you mean, who can you force it to use discrete values rather than continuos?
place a knob control on the diagram , right click it and choose representation, and an integer datatype. this will force it to be discrete values. if you only want three different values, and these are arbitary, click on the first value with the text tool, and change it to 1, and on the last one to change it to 3.
if the values matter, right click on the knob and use scale, marker spacing, arbitary, and then scale, add marker and remove marker untill you have enough markers. reset the scale to uniform, and then edit the values so that they are what you want.
Hope this helps.
Matt