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For a given integer $n$ and a set $ \mathcal{S} \subseteq \mathbb{N} $, denote by ${ R}_{h, \mathcal{S} }^{(1)} (n)$ the number of solutions of the equation $n= {s}_{{i}_{1} } + \cdots + {s}_{{i}_{h} } $, ${s}_{{i}_{j} } \in \mathcal{S} $, $j= 1, \ldots , h$. In this paper we determine all pairs $( \mathcal{A} , \mathcal{B} )$, $ \mathcal{A} , \mathcal{B} \subseteq \mathbb{N} $, for which ${ R}_{3, \mathcal{A} }^{(1)} (n)= { R}_{3, \mathcal{B} }^{(1)} (n)$ from a certain point on. We discuss some related problems. [ABSTRACT FROM AUTHOR]