Displaying Drop Down Field Conditionally

I am learning Appian now.
In my UI, secondColumnContents contains 3 dropdownFields. But one of dropdownFields only be displayed under one condition. How can I display this dropdownField under the condition. So, the UI may display 3 dropdownFields if the condition is satisfied. Otherwise, it will display 2 dropdownFields. Could any one show me an example. Thanks.

OriginalPostID-221194

OriginalPostID-221194

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