Superconformal N=3 SYM low-energy effective action

We construct a manifestly N = 3 supersymmetric low-energy effective action of N - 3 super Yang-Mills theory. The effective action is written in the N = 3 harmonic superspace and respects the full N - 3 superconformal symmetry. On mass shell this action is responsible for the four-derivative terms in the N = 4 SYM effective action, such as F-4/X-4 and its supersymmetric completions, while off shell it involves also higher-derivative terms. For constant Maxwell and scalar fields its bosonic part coincides, up to the F-6/X-8 order, with the bosonic part of the D3 brane action in the AdS(5) x S-5 background. We also argue that in the sector of scalar fields it involves the correctly normalized Wess-Zurnino term with the implicit SU(3) symmetry.