Assuming there aren’t too many performance requirements for this, and also assuming you’re going to need to do this more than once:
var src_list = [array_one, array_two, array_three]
var dst_list = []
func sort_by_size(a, b) -> bool:
if a.size() < b.size():
return true
return false
func build_list() -> void:
src_list.sort_custom(sort_by_size)
for el in src_list:
dst_list.append_list(el)
That basically puts all your small lists inside a bigger list where we can sort them by size with sort_custom(). Once we have that, we iterate over them, expanding them into dst_list.