Finding a square root of a 3x3 upper triangular matrices is not hard. Assuming that the entries on the diagonal of A are real and real and positive,define the 3x3 matrices as follows. For let . Also define
Let and
See that of course, this will also work for matrices with complex entries if you choose a square root for each and if the denominators in and are non-zero. Since every square matrices with complex entries is similar to an upper triangular matrices, you can use the above to find a square root of an arbitrary 3x3 matrices.