If you look at the geometry of the J-Roberts or the Brodak 3-wire bellcrank, you'll see (after some hard thinking) that all the various lever arms are arranged so that each line carries 1/3 of the total line tension.
If you have one handy you can test this -- when you move the throttle line outward, the up and down lines pull inwards about 1/2 as much, and visa-versa.
This is necessary. Because you can't push on a rope, in order for the throttle "signal" to get transmitted to the plane via the line position, there must be tension on all the lines. For everything to work nicely, this tension needs to be evenly distributed.