The k3tog, or knit 3 together, is a right-leaning double decrease. It takes three stitches and turns them into a single stitch that slants toward the right.
To work it, insert your right-hand needle into the first three stitches on the left-hand needle at the same time, just like you would to knit a single stitch. Catch the working yarn with the tip of the right-hand needle and draw this new stitch through all three of the stitches on the left-hand needle. Finally, slide those three stitches off the needle.
The following video demonstrates how to work a k3tog. English style is shown starting at 0:34, and Continental starts at 1:29.