The following result is proved. Theorem: Let \((S,+)\) be a commutative semigroup and let \(X\) be a~sequentially complete linear topological Hausdorff space. Assume that \(V\) is a sequentially closed, bounded, convex and symmetric with respect to zero subset of \(X\). For arbitrary functions \(f,g,h:S\to X\) satisfying the condition \[ f(x+y)-g(x)-h(y)\in V,\quad x,y\in S, \] there exist the functions \(f_1,g_1,h_1:S\to X\) such that \[ f_1(x+y)-g_1(x)-h_1(y)=0,\quad x,y\in S, \] \(f_1(x+y)-f(x+y)\in 15V\), \(g_1(x)-g(x)\in 7V\) and \(h_1(x)-h(x)\in 7V\) for all \(x,y\in S\).