The number of values of λ for which there exist a non-zero 3x3 matrix A such that A′=λA is :
0
1
2
infinite
We have
A=A′′=(λA)′=λA′=λ2A.
As A≠0,λ2=1⇒λ=±1.
For λ=1 all non-zero symmetric matrices can work as A.
For λ=-1, all non-zero skew symmetric matrices can work as A.