You don't have to have the sts on the correct number needle but it sure makes the decreases easier to remember. You should have 3 heel sts plus 8 on needle 1, 12 instep sts on needle 2, and 8 plus 3 heel sts on needle 3. You're next going to decrease by k2tog, 3 sts before the end of needle 1 and then start off needle 3 with a k1, slip 1, k1, psso for several rounds. Having the correct number of sts on the 3 needles keeps those decreases in the right place. You could use st markers, but I think this is easier.