A matrix is diagonalizable if it is similar to a diagonal matrix. For an n x n diagonalizable matrix, the sum of the dimensions of its eigenspaces must equal n.
If a matrix is 4 x 4, the sum of the dimensions should be 4 for it to be diagonalizable. It is given that the matrix has 3 eigenvalues which means that to be diagonalizable, one of the eigenvalues is a double eigenvalue. A will not be diagonalizable if this double eigenvalue has only one independent eigenvector. However, it is given that one of the eigenspaces is two-dimensional - this will be from the double eigenvalue! The other two eigenvalues will contribute single eigenvectors. The these will provide the potential basis of eigenvectors of `R^4` .
A related question is provided below as a reference.