Override custom CSS for a single page

Hi guys,

I have a question regarding overriding custom CSS only for a single page.

I’ve already updated the custom CSS so that the content area is 800px, which worked fine. The codes are as follow:

.entry-header,
.entry-content,
.entry-summary,
.entry-meta p {
text-align: justify;
max-width: 800px;
margin: auto
}

However, there is this page which I want to make use of the default width in content area so I can pull this off:

Header image
Text
Text column 1|Text colum 2|Textcolum 3
Long text
Text column 1|Text colum 2|Textcolum 3 again

So here are my questions:
1. Could you tell me what should I do to revert the width back only in this page (while the other pages and posts still 800px)?
2. What should I do to divide the pages into 3 columns and back and 3 columns again? Is there any other way beside using <div class=”col-md-x”></div>?