The third-preferred, or tertiary, function tends to be less interesting to individuals, and they tend to have fewer skills associated with it. The letter of this function does not appear in your type. It is the opposite function from the auxiliary function. If, for example, your auxiliary function is Thinking, then your tertiary will be Feeling.
Development of this function tends to come later in life (about midlife) after you have grown and feel comfortable with the dominant and auxiliary. As you grow and develop, you learn that there is a time and place to use your third and fourth functions.
About this time, the question arises in life, is this all there is? The tertiary function can guide you toward areas of your life you have avoided, areas that require skills you do not feel comfortable using. For example, a Thinking type with tertiary Intuition may begin taking literature courses. A Thinking type with tertiary Sensing may begin doing carpentry or weaving.