You can find A and B with two simultaneously equations.
For the first equation assume uniaxial tension yielding - i.e. take σ1 as fty and σ2=σ3=0.
For the second equation assume uniaxial compression yielding. This is a polymer so it makes sense that fty and fcy would be different values - and they have to be different values otherwise you'd find that A=0.
Once you have A and B, then consider the hydrostatic case - i.e. take σ1=σ2=σ3=σ. Solve for σ.