You're going to have 3 sections, each with their own strand of yarn, or you can do each one separately.
knit the first set of stitches, drop the yarn and pick up another end of yarn, just start working with it. BO for the first leg opening, then continue knitting across the back with it. Drop that yarn, use another end (it can be from the other end of the other ball if you like) BO sts for the second leg opening and knit to the end of row. Turn to knit the WS row, when you get to the bound off stitches, drop that yarn, pick up the one on the other side of the bound off ones and knit the middle section. Then drop the yarn and pick up the 3rd end and do the last set of stitches. Turn and do the RS row, repeat until the right length and finish a WS row. The next row will probably have you knit all the stitches with just the first strand of yarn again and Cast on sts over the bound off ones.
sue- knitting heretic