how to make three probability vectors to plot entropy

Hi, i have to make three vectors with probabilities p1, p2 and p3= (1-p1-p2) in order to plot the entropy of a signal source without memory that produces three symbols. I have tried many things with rand() and vectors like [1: .001:1] but none worked as it supposed to, the main problem is that i can't maintain a constant sum of one for every set of probabilities. Is there any way or funtion to do this? Any piece of advice would be appreciated.