triangle2 huge *tri;
tri=(triangle2 huge*)farcalloc(numtri,sizeof(triangle2));

but now i have to  create an array of pointers being able to hold
(in the worst case) all of the triangles for depht sorting them.
so how do i create an array of (huge) pointers which i can set
to different triangles? it has to be dynamically allocated just as the
triangles, as their number varies.

triangles = malloc (maxnumtris * sizeof *triangles);

With the standard functions, there is no need to cast the pointer returned,
as it is a void pointer and can be assigned to any object pointer without a
cast.

Now you have a dynamically created array-like object that consists of
pointers, ones that point nowhere and can't be used yet.

Now, when you need to create a specific instance of a triangle you can use
array notation to access it. You must allocate storage for that triangle
before you do anything else with that pointer.

triangles[3] = malloc (sizeof *triangles[3]);

I'm not entirely sure that such a data structure is best for your problem.
I'd think a tree would be better, but without more information on what you
are doing, it's hard to say, and perhaps off topic anyway.

You now say you want to allocate space for numtri objects of type
trianlge2*.

The process is exactly the same.  To dynamically allocate an array of
type X, you declare a pointer to type X and allocate space for (x)
objects of type X.  In this case, X is pointer to triangl2.  Not
knowing anything about huge or far, I suggest you try something along
the lines of

        triangle2 **tri_p;
        tri_p = calloc(numtri, sizeof *tri_p);
        for (i = 0; i < numtri; i++)
                tri_p[i] = &tri[i];

triangle2 *tri;
tri = calloc(numtri, sizeof *tri);