There are 3 choices for the pants. Having selected a pair of pants there are 2 choices for a shirt. Therefore there are 3 * 2 possible choices for a pair of pants and a shirt. Each of these 6 possible choices can be combined with each of the 3 possible choices of a pair of shoes. So, the total number of choices of outfit is given by:
[tex]3\times2\times3=18\ choices[/tex]