assign_3 assigns the value of one existing triple-intr to another
triple-intl.
assign_int_3 and assign_unsigned_3 assigns the argument r to
the least significant position of the triple-intl. assign_int_3
extends the sign, while assign_unsigned_3 does not.
RETURN VALUE
assign_3, assign_int_3 and assign_unsigned_3 all return a pointer
to the freshly assigned triple-intx.