k3tog (knit three stitches together) is a multiple decrease, and decreases the stitch count by two stitches.
There are a few ways to k3tog, so if your knitting pattern has instructions on how to make the decrease then do follow the pattern.
If your pattern does not give instructions, this tutorial shows how to work a balanced k3tog decrease. This is a balance decrease because it doesn't slope only to the left or only to the right. As you can see in the photo this k3tog has the centre stitch of the decrease on top, and the knitting slopes left and right at either side.
How to work k3tog
Step 1: Put the tip of the right hand needle knitwise into the second and then the first stitches on the left needle (as if you were going to knit 2 together).
Step 2: Slip the two stitches onto the right hand needle.
Step 3: Knit the next stitch on the left hand needle as normal.
Step 4: Lift the two slipped stitches on the right hand needle over the knitted stitch on the right hand needle to finish the decrease.
And you are done!