Hey There,
The borders of the last image is the result of the uncleared floats used int the first two image. You may update your shortcodes and use this:
[caption class="caption-1" id="attachment_1" align="alignright" width="300"]<img src="http://theseforeignroads.com/wp-content/uploads/2018/04/Blog-Central-America-Steep-Road.jpg" alt="Kanagawa" title="The Great Wave" width="300" height="205" class="size-medium wp-image-6" /> The Great Wave Caption 1[/caption]
[caption class="caption-2" id="attachment_2" align="alignright" width="300"]<img src="http://theseforeignroads.com/wp-content/uploads/2018/04/Blog-Central-America-Steep-Road.jpg" alt="Kanagawa" title="The Great Wave" width="300" height="205" class="size-medium wp-image-6" /> The Great Wave Caption 2[/caption]
<div class="clear" style="clear: both"></div>
[caption class="caption-3" id="attachment_6" align="alignnone" width="300"]<img src="http://theseforeignroads.com/wp-content/uploads/2018/04/Blog-Central-America-Steep-Road.jpg" alt="Kanagawa" title="The Great Wave" width="300" height="205" class="size-medium wp-image-6" /> The Great Wave <br><small>The small caption text</small>[/caption]
As you noticed in the last of the caption shortcode, I have this The Great Wave <br><small>The small caption text</small>
. This would create a two rows of text. The second one will be smaller font. You can also define the font size of the texts by using an inline css like this:
<span style="font-size: 22px;">The Great Wave</span> <br><span style="font-size: 14px;">The small caption text</span>
And you will have something like this as a result:
Hope this helps.