Here's an idea.
1. Setup 3 different rules that identify the IHC payer account number in "Define Route Processing" e.g.
Route - BNK1 ; Rule - BNK1; Payer Account Number - SUB1, SUB2, SUB3
Route - BNK2 ; Rule - BNK2; Payr Account Number - SUB4, SUB5, SUB6
2. In make basic settings for payment process, Routes - assign each route to a different clearing partner; you'll also need the payment method data configuraiton in this node as well
3. Then in Setup creation of payment requests for inbound IDoc in FI, define the payment details for each clearing partner.
I think that should give you what you need. Unless you have other specific rules that you might need in the routing.
-Anya