jQuery slider for WordPress
Here's how I would have written the slider. For a small project like this, you probably don't need to use Prototypal Inheritance. However, if you wanted to scale or add new functionality it would be a walk in the park. As you dive into larger projects, taking time to plan how you're going to structure your code will greatly improve your ability to write the same. Keep in mind if any part of this is confusing or unclear, I'll be happy to walk you through it. I've included the code in this fiddle, but I'll do most of the explaining in the commented code below.
Now onto the fun stuff. You can place the following code in a separate .js file.
function Slider ( container, nav ) {
    //Get the elements passed in
    this.container = container;
    this.nav = nav;

    this.divs = this.container.find('div'); //Select the divs in the container
    this.divWidth = this.divs.width(); //Width of each div
    this.divsLen = this.divs.length; //Count how many divs there are

    this.current = 0; //Set the counter to the first div
};

//This function does all the animation
Slider.prototype.transition = function( speed ) {
    this.container.animate({
        //Set the margin to a negative, hence the &quot;-&quot; in front.
        //Get the current div, multiply by its width, and multiply by 3.
        //This makes the animation move three divs at a time.
        //If you changed the width in the CSS, this code would work just fine.
        'margin-left': -( this.current * this.divWidth * 3 )
    }, speed); //Here I get the speed from the click event set up in your html file.
};

//Here is where it might get tricky. 
//This function controls the current set of divs being displayed.
Slider.prototype.setCurrent = function( dir ) {
    var pos = this.current; //Cache the current div

    //Okay. Take a breather right here :)
    //Get the current div and add to it depending on what is returned.
    //We test to see if the clicked button was 'next'.
    //If true, it returns '1' and adds that to 'pos'.
    //If false, that means you clicked 'prev'. It then returns a '-1' and adds to 'pos'.
    pos += ( ~~( dir === 'next' ) || -1 );

    //Now if we kept clicking 'prev', we would end up with a negative number after a while.
    //This tests to see if 'pos &lt; 0'.
    //If yes, take the total divs, divide by 3, and subtract one.
    //This will make it display the last three divs. It kinda goes 'backwards'.
    //If pos isn't &lt; 0, then take modulus of pos and the total / 3
    //Modulus is a math operator and if you're not clear on it, there's a link in the bottom.
    this.current = ( pos &lt; 0 ) ? (this.divsLen / 3) - 1 : pos % (this.divsLen / 3);

    return pos;
};

This part comes after your script in the previous .js file. It can be in your html page at the bottom. I set up the previous piece of code so that we can refrain from referencing any DOM elements or specific values, keeping the logic separate. This part now sets all the configurations we and values to be sent to the slider.
//First off, if your user doesn't have JS on, your plugin will fail.
//I've set the wrapper overflow to scroll in the CSS, so if there is no JS, the
//user can still scroll and see all the divs.
//Here we set the overflow to hidden and select our container element.
var container = $('.wrapper').css('overflow', 'hidden').children('.container');

//Set up the Slider constructor function, and pass in the elements we want.
var slider = new Slider( container, $(&quot;.nav_wrapper&quot;) );


//Again, if there's no JS, there's no point in having buttons.
//The buttons are hidden in the CSS, and here we display then with .show().
//Then we select and set up the click event on the actual div button.
slider.nav.show().find('div').on('click', function() {
    //When clicked, we call the functions from our previous code, passing in our configurations.
    //By doing this, if we want to change anything, it can be done here and only done once.
    //Calls setCurrent and passes the clicked div class. It's either 'prev' or 'next'.
    slider.setCurrent( $(this).attr('class') );

    //After the current div is all figured out, we just animate.
    //Call transition and pass in the speed we want.
    slider.transition( 1000 );
});

Hopefully this all made sense to you. If you're a beginner in JS, this might be a little over your head, but that's perfectly ok! Try reading as many articles as you can find on the subjects here and you'll catch on soon.
Article explaining modulus: http://mathforum.org/library/drmath/view/54363.html