$解:(3)\because FH是四邊形EFGH的“相似對角線”,$
$\therefore \triangle EFH∽\triangle HFG,$
$\because \angle EFH=\angle HFG,$
$\therefore \triangle FEH∽\triangle FHG,$
$\therefore \frac{FE}{FH}=\frac{FH}{FG},$
$\therefore FH^{2}=FE\cdot FG,$
$如圖,過點E作EQ\bot FG于Q,$
$\therefore EQ=FE\cdot \sin 60^{\circ}=\frac{\sqrt{3}}{2}FE,$
$\because \frac{1}{2}FG?EQ=2\sqrt{3},$
$\therefore \frac{1}{2}FG×\frac{\sqrt{3}}{2}FE=2\sqrt{3},$
$\therefore FG\cdot EF=8,$
$\therefore FH^{2}=FE\cdot FG=8,$
$\therefore FH=2\sqrt{2}$