Is it possible to add a new record to the parent table and then add a new record to the child table and then add a new record to the grandchild table from inside one layout?

Parent: Students

Child: Admission

Grandchild: Agency Admission

I have another table called Agencies and the Agency Admission is a join table between Agencies and Admissions. One student can have multiple admissions and multiple Agencies can be involved in one admission.

Any help is greatly appreciated. Thanks.

It's not possible to have a portal in a portal. So, I would probably have a button on Student "New Admission" which creates the new Admission record and navigates to its form view. On the Admission form view, I would have a portal to the "Agencies Involved" - your AgencyAdmission join table. If you only have a limited number of Agencies from which to choose, I'd use a popup menu in the portal of AgencyID Name. If you have more than 10 Agencies, use a popup select window.