How to add in-direct related Records to a sub-grid on a form.

Hello All,

I am having 3 entities which are like  A(Nto1)--->B<----(Nto1)C. There is no direct relation between A & C. But when user creates a new Entity 'A' record, he has to select record B( from drop down ) and when user saved that record , there is a requirement to auto populate all related C entity records(related to B) on form A sub-grid. I'm thinking to write plug-in, but i'm not sure how to add to the sub-grid. please let me know if there's any simple way to achieve this . Thank You