How to Adjust a bill (add to bill) on a client with a billing rule (1/3, 1/3, 1/3) but not add to others that share rule

I have a bill that needs to be adjusted.  I a billing rule set up to charge the 3 associated bills (1/3, 1/3, 1/3).  On one of the bills I need to add a credit (which I was able to do with no problem), but on the other two associated bills, I need to add an amount to the bills (that won't be shared) amongst all associated clients.  However, all other entries still need to be shared per original rule of 1/3, 1/3, 1/3.

Any ideas? 

Thank you!

Janelle