The sab2gs filter can be used convert SAB (or STB) arrangement of parts on three staves into a two-staff grand staff arrangement.
The middle voice will be associated with the top staff. To move
the middle voice to the bottom staff, add a *clefF4 interpretation.
Then to move back to the top staff by using *clefG2 in the middle
voice.
An alternate method that can be used by itself or by mixing with
the clef method: use *down to move the middle voice to the bottom
staff, and *Xdown to move back to the top staff. This allows the
middle staff to have its own clefs in open-score format.
Here is an example of processing a Bach three-part invention. In
this case the bottom staff changes to *clefG2, requiring using
*down to move the middle voice to the bottom staff (although
*clefF4 could be used, but it would not look good in open-score
format being displayed in the wrong clef.