To reestimate stitches per inch when double stranding a yarn, you need to multiply by 0.7. But what about if you want to triple strand? Marnie says in her patternthat if you wanted to hold three or more strands of sport weight yarn together at once–as long as it equals gauge (14 sts x 20 rows = 4" on size 10½ [6.5mm])–you can do that to use for the yarn for this hat. I thought it might make a cool effect rather than just using one color of one strand of a bulky weight yarn, but I’m not sure how I’d figure the gauge for a triple strand of sport weight yarn held together.
That might be doubled, but about the only thing you could do would be to knit up a sample and see what you get.