Combining border-top,border-right,border-left,border-bottom in CSS

Your case is an extreme one, but here is a solution for others that fits a more common scenario of wanting to style fewer than 4 borders exactly the same.

border: 1px dashed red; border-width: 1px 1px 0 1px;

that is a little shorter, and maybe easier to read than

border-top: 1px dashed red;  border-right: 1px dashed red; border-left: 1px dashed red;

or

border-color: red; border-style: dashed; border-width: 1px 1px 0 1px;

I can relate to the problem, there should be a shorthand like...

border: 1px solid red top bottom left;

Of course that doesn't work! Kobi's answer gave me an idea. Let's say you want to do top, bottom and left, but not right. Instead of doing border-top: border-left: border-bottom: (three statements) you could do two like this, the zero cancels out the right side.

border: 1px dashed yellow;
border-width:1px 0 1px 1px;

Two statements instead of three, small improvement :-D

No, you cannot set them all in a single statement.
At the general case, you need at least three properties:

border-color: red green white blue;
border-style: solid dashed dotted solid;
border-width: 1px 2px 3px 4px;

However, that would be quite messy. It would be more readable and maintainable with four:

border-top:    1px solid  #ff0;
border-right:  2px dashed #f0F;
border-bottom: 3px dotted #f00;
border-left:   5px solid  #09f;