It's a many to many table

TableB ( idA, idB )

TableC ( idA, idC )

Why I can't have

TableManyToMany ( idA, idB, idC )

why I need to have

TableManyToMany ( idAb, idB, idAb, idC )

PS: I tried the IRC and mailing lists, but noone helped in any case. 
I need to know what the hell is this problem to solve it.