3rd - Tertiary

The Tertiary Function, (also called as the child function) is our third function. It is the function we are comfortable messing up, being childish with. We're not really that competent or great with this function (though can be developed), but we are optimistic and comfortable with not being overly competent at using it and we're okay when people make fun of us for it. The tertiary function is always the opposite of our auxiliary function, so if your auxiliary function is Ni, your tertiary function is Se, and if your auxiliary function is Se, your tertiary function is Ni. 

According to Carl Jung, the tertiary function starts to appears in ours twenties or early thirties. (Remember that this is just an approximate, everyone matures differently)

Examples:

Fe Tertiary: When someone's Fe is at their tertiary function, they childishly use it, meaning their preference isn't so much of that of community/shared values, but it is comfortable and fun for them to utilize, and can result to them provoking external emotional responses. (due to childishness utilization of Fe)

Ni Tertiary: When someone's Ni is at their tertiary function, they childishly use it, meaning their preference isn't so much of that of symbolic or abstract perceptions, however they can childishly use it for fun, like learning astrology for fun. 

Sources: https://presentobsessions.wordpress.com/2013/12/03/the-john-beebe-8-function-model/