Let \(G\) be an \(n\)-divisible abelian group and \(X\) a normed space. A mapping \(f: G\to X\) is additive if and only if it is a solution of the functional equation
\[ f(x)+f(y)+nf(z)=nf\left(\frac{x+y}{n}+z\right),\qquad x,y,z\in G\tag{1} \]
and if and only if it is a solution of the functional inequality
\[ \|f(x)+f(y)+nf(z)\|\leq\left\|nf\left(\frac{x+y}{n}+z\right)\right\|,\qquad x,y,z\in G.\tag{2} \]
For \(n=2\), the stability of (1) and (2) is proved. Namely, assuming, respectively,
\[ \left\|f(x)+f(y)+2f(z)-2f\left(\frac{x+y}{n}+z\right)\right\|\leq\varphi(x,y,z),\qquad x,y,z\in G\tag{3} \]
or
\[ \|f(x)+f(y)+2f(z)\|\leq\left\|2f\left(\frac{x+y}{n}+z\right)\right\|+\varphi(x,y,z),\qquad x,y,z\in G,\tag{4} \]
and with suitable assumptions on the control mapping \(\varphi: G^3\to\mathbb{R}^{+}\), it is proved that \(f\) can be approximated by a unique additive mapping. If \(G\) is a normed space, the results can be applied in particular to control mappings of the form \(\varphi(x,y,z)=\theta(\|x\|^p+\|y\|^q+\|z\|^t)\) or \(\varphi(x,y,z)=\theta\|x\|^p\|y\|^q\|z\|^t\). In the latter case one gets the superstability effect, i.e., (3) or (4) yield that \(f\) itself is additive.