On a circle, $n \geq 3$ points are marked. Each marked point is coloured red, green or blue. In one step, one can erase two neighbouring marked points of different colours and mark a new point between the locations of the erased points with the third colour. In a final state, all marked points have the same colour which is called the colour of the final state. Find all $n$ for which there exists an initial state of $n$ marked points with one missing colour, from which one can reach a final state of any of the three colours by applying a suitable sequence of steps. 
1. **Initial Setup and Definitions:**
   - Let the three colors be denoted as \(1, 2, 3\).
   - We are given \(n \geq 3\) points on a circle, each colored either red, green, or blue.
   - In one step, we can erase two neighboring points of different colors and mark a new point between the locations of the erased points with the third color.

2. **Claim for Even \(n\):**
   - If \(n\) is even, we can reach a final state where all points are of the same color.
   - Suppose we start with an initial state where one color is missing, say color \(3\). Thus, the points are colored only with \(1\) and \(2\).

3. **Transformation Process for Even \(n\):**
   - Consider an arc of the circle with points colored as \(1, 2, 1, 1, \ldots, 1\) (with \(2k-2\) points in total).
   - By performing the operations from left to right, we can transform this sequence into a single point of color \(3\).
   - This process can be repeated for any pair of neighboring points with different colors, eventually leading to a state where all points are of the same color.

4. **Proof for Odd \(n\):**
   - Assume \(n\) is odd and that we start with an initial state where one color is missing, say color \(2\).
   - The operations allowed are:
     \[
     1, 2 \rightarrow 3, \quad 1, 3 \rightarrow 2, \quad 2, 3 \rightarrow 1
     \]
   - Notice that each operation changes the total number of points of colors \(1\) and \(3\) by \(0\) or \(2\).
   - Since \(n\) is odd, the total number of points of colors \(1\) and \(3\) is also odd.
   - Therefore, at any step, the number of points of colors \(1\) and \(3\) remains odd.
   - This implies that it is impossible to reach a state where all points are of color \(2\) (since the number of points of colors \(1\) and \(3\) would need to be zero, which is even).

5. **Conclusion:**
   - For even \(n\), it is possible to reach a final state where all points are of the same color.
   - For odd \(n\), it is not possible to reach a final state where all points are of the same color if one color is missing initially.

The final answer is \( \boxed{ n } \) must be even.