Adding a third UD table

Vantage 8.03.408A Progress

I have a custom form set up for UD105 & UD105a tables. These are by nature a parent child relationship. I now want to add a third table (UD15) to be the child of UD105a. What is the best (easiest?) way to do this? I need to be able to add, delete, and change UD15 on the UD105/UD105a form.

Steven G.