Let $ABC$ be an arbitrary triangle. A regular $n$-gon is constructed outward on the three sides of $\triangle ABC$. Find all $n$ such that the triangle formed by the three centres of the $n$-gons is equilateral.
1. **Define the problem using complex numbers:**
   Let \( A = a \), \( B = b \), and \( C = c \) be the complex numbers representing the vertices of triangle \( \triangle ABC \). Let \( O_1, O_2, O_3 \) be the centers of the regular \( n \)-gons constructed outward on sides \( AB \), \( BC \), and \( CA \) respectively. We need to find \( n \) such that the triangle formed by \( O_1, O_2, O_3 \) is equilateral.

2. **Express the centers of the \( n \)-gons using complex numbers:**
   Let \( \alpha \) be the \( n \)-th root of unity. The centers \( O_1, O_2, O_3 \) can be expressed using the following relations:
   \[
   \frac{O_1 - a}{O_1 - b} = \alpha, \quad \frac{O_2 - b}{O_2 - c} = \alpha, \quad \frac{O_3 - c}{O_3 - a} = \alpha
   \]

3. **Condition for \( O_1O_2O_3 \) to be equilateral:**
   For the triangle \( O_1O_2O_3 \) to be equilateral, the sum of the complex numbers representing the vertices, rotated by \( \omega \) (the third root of unity), must be zero:
   \[
   O_1 + O_2 \omega + O_3 \omega^2 = 0
   \]

4. **Substitute the expressions for \( O_1, O_2, O_3 \):**
   Using the given relations, we substitute \( O_1, O_2, O_3 \) into the equilateral condition:
   \[
   \frac{O_1 - a}{O_1 - b} = \alpha \implies O_1 = \frac{a \alpha - b}{\alpha - 1}
   \]
   \[
   \frac{O_2 - b}{O_2 - c} = \alpha \implies O_2 = \frac{b \alpha - c}{\alpha - 1}
   \]
   \[
   \frac{O_3 - c}{O_3 - a} = \alpha \implies O_3 = \frac{c \alpha - a}{\alpha - 1}
   \]

5. **Simplify the equilateral condition:**
   Substitute these expressions into the equilateral condition:
   \[
   \frac{a \alpha - b}{\alpha - 1} + \frac{b \alpha - c}{\alpha - 1} \omega + \frac{c \alpha - a}{\alpha - 1} \omega^2 = 0
   \]
   Factor out \( \frac{1}{\alpha - 1} \):
   \[
   \frac{1}{\alpha - 1} \left( (a \alpha - b) + (b \alpha - c) \omega + (c \alpha - a) \omega^2 \right) = 0
   \]
   Since \( \alpha \neq 1 \), we have:
   \[
   (a \alpha - b) + (b \alpha - c) \omega + (c \alpha - a) \omega^2 = 0
   \]

6. **Solve for \( \alpha \):**
   To satisfy this equation, we need:
   \[
   a \alpha + b \alpha \omega + c \alpha \omega^2 = b + c \omega + a \omega^2
   \]
   Factor out \( \alpha \):
   \[
   \alpha (a + b \omega + c \omega^2) = b + c \omega + a \omega^2
   \]
   For this to hold, \( \alpha \) must be \( \omega \), the third root of unity. Therefore, \( n = 3 \).

The final answer is \( \boxed{ n = 3 } \).