We find double-extreme black holes associated with the special geometry of
the Calabi-Yau moduli space with the prepotential F=STU. The area formula is
STU-moduli independent and has [SL(2,Z)]3 symmetry in space of charges.
The dual version of this theory without prepotential treats the dilaton S
asymmetric versus T,U-moduli. We display the dual relation between new (STU)
black holes and stringy (S|TU) black holes using particular Sp(8, Z)
transformation. The area formula of one theory equals that of the dual theory
when expressed in terms of dual charges. We analyse the relation between (STU)
black holes to string triality of black holes: (S|TU), (T|US), (U|ST)
solutions. In the democratic STU-symmetric version we find that all three S and
T and U duality symmetries are non-perturbative and mix electric and magnetic
charges.Comment: 12 pages, 2 Postscript figures, ref.added, minor corrections, version
to appear in Phys. Rev.
