Anwer to your original question:

#LeftPane{
    background-color:orange;
    float: left;
    width: 20%;
    margin-left: 2px;
}
#ContentPane{
    background-color: purple;
    float: left;
    width: 59%;
    margin-left: 2px;
}
#RightPane{
    background-color:teal;
    float: left;
    width: 20%;
    margin-left: 2px;}

For a fixed with skin I wouldn't use the precentages, but hard coede widths

Yes, I have done tableless skins (but some times I also use an occasional table, but I prefer not to)

You target the contenpane with id "dnn_contenpane" in CSS

Yes I have done some 3 collumn layout without tables.

It's doable but you need some CSS experience...(and tables are a lot faster to do)

It's not that difficult to create a tableless static page, the problem is that since DNN is a CMS the size of the content changes all the time, that makes it difficult.

You will find out that if you created your skin the appearing control panel at the top can completely rip your skin apart.

I think the main reason people are still using tables for layout is that CSS2.1 is just not good enough in providing DIV alternatives for Tables. For years the browser support for CSS2.1 was the problem, now the standard is the problem IMHO.
We'll just have to wait 5 more years for CSS 3 (and 5 more years for every browser to support it)

You might have a look at this great page with CSS examples