By trial and error we can see that one root is x=3 so (x-3) is a factor. Divide the polynomial by this factor and we get x^2-2x+2. Use the quadratic formula to find the remaining zeroes: x=(2+sqrt(4-8))/2=(2+sqrt(-4))/2=(2+2i)/2=1+i. So the three roots are 3, 1+i, 1-i.
(I used synthetic division to divide the known factor into the polynomial, but algebraic division could also have been used.)