First off, they're not strings, they would be strands of yarn. And you don't need a special pattern necessarily if you want to knit with 3 held together, just treat them as if they were 1 strand and knit on huge needles. If you mean 3 different colors and want to knit a few rows with each color, that's stripes. Knitting a picture with 3 colors is called intarsia, and knitting a piece with 3 colors in a pattern - a few stitches with one color, the next sts with another color - is called stranded or fair isle. So there could be at least 6 answers to your question without more information. If you have a picture of what you mean, a link to it would be helpful.