Finding spectral radius of matrix without computing characteristic polynomial
Solution 1:
I found a solution, I post it in order to mark the question as answered.
As shown here, since $\sum_{i=1}^3a_{ij}=4$ for $j=1,2,3$, we have that 4 is eigenvalue of $A$. Therefore, $$\rho(A)\geq4$$ and we conclude $\rho(A)=4$.