Research fonts with minimum 3 Variants

feature calt { 

lookup calt_one {

sub @one @one' @one by @two;

sub @two @one' @one by @three;

sub @three @one' @one by @four;

} calt_one;

lookup calt_two {

sub @four @one @two' by @three;

sub @three @three' by @two;

sub @two @four @one' by @three;

sub @four @three @two' by @one;

sub @three @one @three' by @four;

sub @four @four' by @two;

} calt_two;

} calt;

I don't know if this is exactly what you're looking for, but my Open Source Joscelyn font (imitating English secretary hand) has three or more variants for a good many lowercase letters, mostly managed by calt. I don't know that the code is the prettiest in the world, but it is thoroughly commented. My historical script fonts mostly work the same way (see e.g. Cerne, and especially the variants for t).