Clean up If or syntax in JavaScript
Here is my slightly "mathematical" version of the function you have posted:

var checkPositionOnLoad = function (e) {
    var currentPos = 0;
    var listItem = document.getElement('li.current');
    var currentSelected = $('list').getChildren('li').indexOf(listItem) + 1;

    currentPos = -300 * (Math.floor((currentSelected + 2) / 3) - 1);

    $('list').setStyle('left', currentPos);
};


I will try to explain how I got to that succinct version.

I removed the duplication and wrote the function as follows:

var checkPositionOnLoad = function (e) {
    var currentPos = 0;
    var listItem = document.getElement('li.current');
    var currentSelected = $('list').getChildren('li').indexOf(listItem) + 1;

    if (currentSelected &lt;= 3) {
        currentPos = -300 * 0;
    }
    if (currentSelected == 4 || currentSelected == 5 || currentSelected == 6) {
        currentPos = -300 * 1;
    }
    else if (currentSelected == 7 || currentSelected == 8 || currentSelected == 9) {
        currentPos = -300 * 2;
    }
    else if (currentSelected == 10 || currentSelected == 11 || currentSelected == 12) {
        currentPos = -300 * 3;
    }
    else if (currentSelected == 13 || currentSelected == 14 || currentSelected == 15) {
        currentPos = -300 * 4;
    };

    $('list').setStyle('left', currentPos);
};


Then I realized you want to figure where a given selected element falls (in which group of triplets that is {1, 2, 3} {4, 5, 6}, etc..). I played around and came up with a mathematical formula:

Math.floor((i + 2) / 3) 


This basically tells you that for i = 7, the selected element falls in the third group {7, 8, 9} which is the third triplet after {1, 2, 3} and {4, 5, 6}

And the left property can be set using:

var currentPos = -300 * (Math.floor((i + 2) / 3) - 1); // (-1 to make it zero-based index)


And that's it.