You can do the 3 sections separately, but you can also work all across the split row, just using a separate yarn end for each section. That will make sure you have the same measurement in each part. So from where you are now, turn and purl to where you bound off, drop that yarn, pick up a new end and purl the back sts to the other leg opening, drop the yarn and pick up a new end (can be the other end of one of the balls you're working with), and purl the last section. Turn and do the same on the knit row - drop the working yarn and pick up the yarn that's attached to the next set of stitches and knit them with it, drop the yarn, pick up the other one and finish the row.
When you get the 1½" knit, end with a WS row and ready to work a knit row and this time you'll cast on across the bound off stitches to join all the stitches back into one piece again.
sue- knitting heretic