If $$\frac{\sin \theta + \cos \theta}{\sin \theta - \cos \theta} = 3$$, then the value of $$\sin^4 \theta - \cos^4 \theta$$ is equal to:

A

$$\frac{1}{5}$$

B

$$\frac{2}{5}$$

C

$$\frac{3}{5}$$

D

$$\frac{4}{5}$$