Try this:
var arr = ["A","B","C","D"];
for (var i=0, len=arr.length; i<len; i++) {
alert(arr.slice(0, 3).join(","));
arr.push(arr.shift());
}
Without mutating the array, it would be
for (var i=0, len=arr.length; i<len; i++) {
var str = arr[i];
for (var j=1; j<3; j++)
str += ","+arr[(i+j)%len]; // you could push to an array as well
alert(str);
}
// or
for (var i=0, len=arr.length; i<len; i++)
alert(arr.slice(i, i+3).concat(arr.slice(0, Math.max(i+3-len, 0)).join(","));