3.如图,在ΔABC\Delta ABCΔABC中,AB=3ADAB=3ADAB=3AD,CE=EDCE=EDCE=ED,设AB→=a→\overrightarrow{AB}=\overrightarrow{a}AB=a,AC→=b→\overrightarrow{AC}=\overrightarrow{b}AC=b,则AE→=(\overrightarrow{AE}=(AE=( )))