The given figure is a square. The diagonals of a square bisects each other. The point where they they bisect each other is the midpoint of the square. Looking at the given square,
AC = BD = Diagonal
Point E = midpoint
Sice AC bisects BD at E, then
BE = DE = AE = CE
Given that AE = 3, then
BE = 3
AC = AE + CE = 3 + 3
AC = 6