  The problem of construction of low-energy effective action in N=3 SYM theory
is considered within the harmonic superspace (HSS) approach. The low-energy
effective action is supposed to be a gauge- and scale-invariant functional in
N=3 HSS reproducing the term F^4/\phi^4 in components. This functional is found
as a scale-invariant generalization of the F^4-term in N=3 supersymmetric
Born-Infeld action.
