I'm guessing you're making gloves, mittens or a keyhole scarf, and you're going to work on only part of your stitches after this row. Do just what it says--knit three, make an increase in the next stitch (if the pattern doesn't specify one, I like make-one increases because they're easy!) then cut the yarn with a few inches of " tail" for later.
Slide the three stitches you knitted and the two from the increase (the original stitch and the new one) onto a different needle, or for that matter a big safety pin, a twist tie or whatever you have handy that won't let them unravel.
After you get them out of the way for safekeeping, go on with the rest of the pattern. You're almost surely going to come back and do something with the set-aside stitches later.