I get $$-\lambda^3+8\lambda^2-19\lambda+12=0$$
Multiply through by -1 (not essential - I just find it easier that way!)
$$\lambda^3-8\lambda^2+19\lambda-12=0$$
If we assume that, because you are doing it by hand, the solutions are integers, then consider
$$(\lambda-a)(\lambda-b)(\lambda-c)$$
The three constants must multiply together such that $$abc=12$$
If they are each integers then we have either 1, 2 and 6 or 1, 3 and 4 (ok a couple of them might have negative signs attached!).
The sum of the three must also be 8, and since 1+2+6 is not 8, and 1+3+4 is, there is a good chance that the three eigenvalues are 1, 3 and 4.
Plugging each of these into the LHS of the first equation above does indeed give zero.