If the pearls are NOT graduated, so you do NOT have an obvious center pearl, you can guestimate how long each strand should be, and start stringing all three on one side of the clasp. As you get towards the end, you can thread a couple pearls on each strand without knotting them, and then thread the silk or Beader's Secret through the other end of the clasp and tie a bow, and try the necklace on. Add or substract pearls and retie the bows and retry on until you like how it lays.
Three strands are very tricky to nest. I agree with pearlescence. HM QEII's triple strand makes me itchy, too. I find the difference in length between the the top two strands may be shorter than the difference in length between the bottom two strands.
On my dog collar, everything nests just right except for ONE strand, and I've never been able to fix it, as I can't tell which pearls are throwing the length off.
You can kind of fudge the length of one strand by using a longer or shorter piece of gimp (French bullion).
However, there's also the issue of how the center pearls line up. If it's important to you to have the center pearls line up, then start knotting from the center and go out in both directions towards the clasp. Then you'll tie bows on both sides of the clasp to try it on.