3.如图,已知四面体ABCDABCDABCD的所有棱长都等于aaa,EEE,FFF,GGG分别是棱ABABAB,ADADAD,DCDCDC的中点.则GF→⋅AC→\overrightarrow{GF}\cdot \overrightarrow{AC}GF⋅AC与EF→⋅BC→\overrightarrow{EF}\cdot \overrightarrow{BC}EF⋅BC分别等于((( )))