assign 1 payment to different invoices

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I had a several thread a while ago, but still I can't sort this out

INV1 = $ 5,-
INV2 = $ 10.-
INV3 = $ 4,-

1 deposit payment = $ 6.-

payment amount needs to be distributed over 3 invoices. The tricky
part is: I want to define how much exactly of the amount paid is being
sent to which invoice.
So in the end I should see that

INV1 has received $ 3.-    (remaining $ 2.-)
INV2 has received $ 2.-    (remaining $ 8.-)
INV3 has received $ 1.-    (remaining $ 3.-)

I tried to:

1) post my payment to my cash account
2) "assign payment" to INV1 (using the entire sum received ($ 6.-)
3) create 3 splits with the intended values in my A/R
4) "assign payment" to the 2 new splits

before Step 5) I have a reduced payment amount ($3.- instead of $6.-)
shown in my customer Report for example

I don't succed assigning the single splits as payments individually. I
only get one single payment out of this so this is actually split in
2. One part (which I define as payment amount) and the 2nd part (which
is the remainder) that is not assigned as a payment

any hack to do such a thing?