Yes, but there's a legal way to do this.

reference the other two areas with offsets relative to the first area

If you cast the pointers to uintptr_t, and perform your arithmetic on uintptr_t and cast your final pointer back (void * ) before using it, what you've done is perfectly legal and safe (albeit weird) since uintptr_t is an unsigned integer type with all the flexibility of unsigned integers, and if x = (uintptr_t)p then it's guaranteed that p == (void * )x.

In other words:

    char * x = malloc(100);
    char * y = malloc(100);
    ptrdiff_t yminusx = y - x;

    x[yminusx] = '\0'

    char * x = malloc(100);
    char * y = malloc(100);
    uintptr_t yminusx = (uintptr_t)(y) - (uintptr_t)(x);

    *(char *)(void *)((uintptr_t)(x) + yminusx) = '\0'