If I were representing you in this audit, I would create a spreadsheet with 4 columns for each credit card - 3 as outlined above - B1, B2, NB and the fourth would be a "sub-total"column.
The first row of the spreadsheet would be the beginning of year balance for each card. The next row would be all of the non-interest and non-fee charges for January split among the three columns. The next row would be the interest and fee charges allocated for that month. I would allocate the fees in the same manner as the interest. (You might have to concede the fee portion, but I would start with it that way). The next line would be the payments made that month-allocated by the total for that column prior to payment, i.e. balances for first card columns 1, 2, and 3 are 500, 1000, 500 and payment is 400, payment would be allocated 100, 200, 100. The next line would be a subtotal as of the end of January's statement. The fourth column would have an entry on this line adding up the other three columns and showing that the total is the same as the ending balance on the statement.
I would repeat this process for each month and for each card.
Then you have a defensible document. You may not win on every item, but there is a logical application of the circumstances.
This is in accordance with Regulations Section 1.163-8T.