The easiest way is to:

1) Journal a reversal to split the Payment into 3 parts.
2) Reconcile the reversal with the original Payment.

Detailed steps:

1) Journal a reversal to split the Payment into 3 parts:

* Visit Accounting --> Adviser --> Journal Entries
* Click CREATE

 

Aging:

* Visit Accounting --> Reporting --> Partner Reports --> Aged Payable

 

2) Reconcile the reversal with the original Payment:

* Visit Accounting --> Adviser --> Journal Items
* Filter for the ACCOUNT and VENDOR

* Select the Item for the DEBIT and CREDIT of $2,100

* From the ACTION menu, select RECONCILE ENTRIES

 

Aging:

Visit Accounting --> Reporting --> Partner Reports --> Aged Payable

 

Now, the three Payments will show below each Vendor Bill for you to apply: