Bootstrap 5 (update 2021)
Since flexbox is still used the centering methods in Bootstrap 5 work the same way. Columns can be centered using offset, auto-margins or justify-content-center (flexbox).
Demo of the Bootstrap 5 Centering Methods
Bootstrap 4 (original answer)
There are multiple horizontal centering methods in Bootstrap 4...
text-center for center display:inline elements
offset-* or mx-auto can be used to center column (col-*)
- or,
justify-content-center on the row to center columns (col-*)
mx-auto for centering display:block elements inside d-flex
mx-auto (auto x-axis margins) will center display:block or display:flex elements that have a defined width, (%, vw, px, etc..). Flexbox is used by default on grid columns, so there are also various flexbox centering methods.
Demo of the Bootstrap 4 Centering Methods
In your case, use mx-auto to center the col-3 and text-center to center it's content..
<div class="row">
<div class="col-3 mx-auto">
<div class="text-center">
center
</div>
</div>
</div>
https://codeply.com/go/GRUfnxl3Ol
or, using justify-content-center on flexbox elements (.row):
<div class="container">
<div class="row justify-content-center">
<div class="col-3 text-center">
center
</div>
</div>
</div>
Also see:
Vertical Align Center in Bootstrap