So far, with the exception of the tiny piece of box girder in the middle of the middle arrowhead in each corner, I believe the box girders are sized to bridge the gaps between each of the hollow concrete columns that hold the roof in place. A vertical spiral column is always placed directly above each column, and the box girder connects the vertical spiral columns.
If you look at the dome, the corner clusters already in place scan 6 columns. There are three columns between the corner clusters.
So my long winded answer to your question is, yes, I think it will take 3 box girders to join the two sections of crown truss together. Looks like there will be one vertical spiral and one diagonal spiral holding these girders up (we only have the one vertical, two diagonal arrow head supports for the corners of the building).