This study explores the Ulam stability of third-order differential equation $h(t)x'''+e(t)x''+f(t)x'+g(t)x=0$ with a new approach, where $e$, $f$, $g$ and $h$ are real-valued variable coefficients. The claim of the main result is that if two differential equations different from the above equation have a complex-valued solution on an interval $I$, then Ulam stability for the above equation is guaranteed on $I$. In addition, an explicit Ulam constant is also derived. The results are applied to Euler differential equations and constant coefficients differential equations.
