Hie ,

Have Three Forms A,  B, C

In Form C have 2 Single Line Fields

In Form B have 2 Lookup Fields , a1 and b1(Based On Form C)

In Form A have Sub-form (Based on Form B)

whenever  any customer fills Form A with Sub-form ,when value of a1 entered, automatically b1 value will auto-populate

Thank you in Advance !!