Triple Z-drive with Stepper Expander

Hi,

I am building a large 3d printer and need triple Z-drive for payload and autolevel reasons. To attach all steppers I use a stepper expander X3 on an RAMPS1.4.
As far as I can see it, Marlin supports dual drive for X,Y,Z, but not triple drive.
Before I start to modify the code, I would like to ask if this was already done in some fork or if I am just missing something in the configuration?

Thanks,
Heiko