Access 兩個subform的問題

我有3個table
A has many B
B has many C

我整左個form , A做main, B同C做subform, 係datasheet形態
原本我係subform B個datasheet上面click一個record,
Subform C個datasheet就會自動show返 對應該個B record的 C records
但係無啦啦呢個對應冇左,subform C 唔識自動
我應該點樣整返出黎