You've been knitting in one piece till now, then you knit some sts, BO a few, continue knitting across the back sts, BO a few and knit the rest of the sts. Think of them as 'sets' of stitches which form different sections. Your piece now looks something like this --
The upper lines are the sts you keep knitting. After the row you BO sts on, turn and knit the first set of stitches, when you get to the gap, drop the yarn, pick up a new end of yarn and knit that set of sts. When you get to the second gap, drop the yarn, pick up another end and knit the last section.
Keep working on each set of sts with it's own yarn for the measurement for the size you're making. Then you work the first set on that row, cast on sts over the gap, work the next set of sts with the same yarn, cast on sts, and do the last set of sts still with the same yarn. That's the 'joining' row, where you get all the sts back into one piece again.
So you don't really need a lot of yarn to do at least one of the set, either use another skein, or the other end of this one, or cut several yards off to use for one of the 3 ends you need. I don't know how big your skein is or if you have more than one, just do what you need to get 3 ends to work with.