  Five different versions of the three-dimensional (3D) reduction of the
Bethe-Salpeter (BS) equation in the instantaneous approximation for kernel of
BS equation for the two-fermion systems are formulated. The normalization
condition for the bound-state wave function in all versions are derived.
Further, the 3D reduction of BS equation without instantaneous approximation
for the kernel of BS equation is formulated in the quasi-potential approach.
Except of the Salpeter version, other four versions have the correct one-body
limit (Dirac equation) when mass of one of constituent fermions tends to
infinity. Application of these versions for investigation of the different
properties of the q bar q bound systems are considered.
