I recently custom made a four strand collar of CF pearls for my daughter's friend. The kind that has two bars and the clasp. I did it on wire because with the knots factored in- there was too much room for error with such an exact fit to the friend's neck. It was 3mm nuggets dyed black, so no no great loss, if it came undone. I fitted each strand as I went, but before I crimped it, I had to take off a pearl here and put one there, even though the count became uneven, the necklace fits as a collar. I think I held my breath the whole time I was fitting it and crimping!
For a longer 3 strander. If you get the first strand the length you want it on wire, then clamp the two sides of the strand together, You have only to make the next strand so it fits to the first one and clamp it so the ends of the strands will match the first strand.
You can use the Bead number wizard to estimate how much longer the knots make the strand.
LINK The rounder the pearls, the better the wizard works.
Here is a link to another thread with a similar question. The photo shows how it would nest better if you use the board. I took four beads off the first strand and added them to the second. It is clear that two beads were all that needed to be switched. For round pearls, that might be the formula. The the bead wizard is there, but not working.