Given a positive integer $n\geq 2$. Let $A=\{ (a,b)\mid a,b\in \{ 1,2,…,n\} \}$ be the set of points in Cartesian coordinate plane. How many ways to colour points in $A$, each by one of three fixed colour, such that, for any $a,b\in \{ 1,2,…,n-1\}$, if $(a,b)$ and $(a+1,b)$ have same colour, then $(a,b+1)$ and $(a+1,b+1)$ also have same colour.
1. **Coloring the First Row:**
   - We start by coloring the first row. Since there are \(n\) points in the first row and each point can be colored in one of three colors, the total number of ways to color the first row is:
   \[
   3^n
   \]

2. **Coloring Subsequent Rows:**
   - Now, consider the second row. For any cell \((2, j)\) in the second row, if it has the same color as the cell \((1, j)\) directly above it, then every cell in this row to the right of \((2, j)\) must have its color fixed based on the cells directly above them.
   - For each \(j\), the rest of the cells to the right of \((2, j)\) must have different colors with respect to the left adjacent cell. This can be done in \(2^{j-1}\) ways.
   - We need to consider all possible \(j\) from \(1\) to \(n+1\), where \(n+1\) corresponds to the case where none of the cells in the second row have the same color as their adjacent left cell. Thus, the total number of ways to color the second row, given the coloring of the first row, is:
   \[
   \sum_{j=1}^{n+1} 2^{j-1} = 2^0 + 2^1 + 2^2 + \cdots + 2^n = 2^{n+1} - 1
   \]

3. **Continuing the Process:**
   - We continue this process for each subsequent row. For each row, the number of ways to color it, given the coloring of the row above it, is \(2^{n+1} - 1\).
   - Since there are \(n-1\) rows left to color after the first row, the total number of ways to color all rows is:
   \[
   (2^{n+1} - 1)^{n-1}
   \]

4. **Combining the Results:**
   - Combining the number of ways to color the first row and the number of ways to color the subsequent rows, the total number of ways to color the points in \(A\) is:
   \[
   3^n \cdot (2^{n+1} - 1)^{n-1}
   \]

The final answer is \(\boxed{3^n \cdot (2^{n+1} - 1)^{n-1}}\)