For a given integer n and a set S ⊆ N, denote by R (1) h,S (n) the number of solutions of the equation n = s i1 + · · · + s ih , s i j ∈ S, j = 1, . . . , h. In this paper we determine all pairs (A, B), A, B ⊆ N, for which R (1) 3,A (n) = R (1) 3,B (n) from a certain point on. We discuss some related problems.2010 Mathematics subject classification: primary 11B34.