Probability of being chosen

Hi there, I need a bit of help on a probability question I was given for homework. I'm getting confused with keeping straight the different formulas. Can you help?

A casting for a play is being held at the local high school's gymnasium. There are a total of 20 actors of differing age groups that attended this casting call; they included 5 children, 7 adults, and 8 seniors. There are only 3 acting roles available for the play and the casting director decided to randomly fill these roles by drawing 3 of the 20 actors' name from a box. Assuming all 3 acting roles in the play are different, what is the probability that 3 actors of the same age range were chosen for the play?