3. $tan^220^0.tan^240^0.tan^280^0=tan^220.tan^2(60^0-20^0).tan^2(60^0+20^0)$$=tan^220.\frac{(\sqrt{3}-tan20^0)^2(\sqrt{3}+tan20^0)^2}{(1+\sqrt{3}tan20)^2(1-\sqrt{3}tan20)^2}$
$=\frac{tan^220^0(3-tan^220^0)^2}{(1-3tan^220^0)^2}=\frac{(3tan20^0-tan^320^0)^2}{(1-3tan^220^0)^2}=tan^260^0=3$