'Centering objects in Rmarkdown slide deck

In this book, section 7.3.3 Content classes show how to provide a two-column layout shown in the code block below:

.pull-left[
- One bullet.

- Another bullet.
]

.pull-right[
![an image](foo.png)
]

How can I do a three-column layout? Something like .pull-center[], which I'm not sure exists.

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