how to assign border to just three sides?

[kh99]

You should download Firefox, if you don't use it already, then install the free add-on called firebug. Then you can load your page, right click on the part you're interested and select "Inspect Element with Firebug". It opens a window that shows the html and what CSS is being applied to it, so you could figure out what CSS selector will work to override it. (I think Chrome has something similar built in, but I don't remember how much it lets you do).

Anyone can do this if they can get to your site, which is why people usually ask for a link for questions like this (it's hard to know exactly what line you want to eliminate just by looking at the picture, but firebug makes it easy). It might sound like a lot of work just to figure out this CSS, but if you like doing your own styles I think you'd find that the time spent figuring it out would pay off in the long run.