I had to play with this in a demo file to make sure I gave good advice.
Make a separate table with one field for each of your three combo boxes. I'll call these three fields Pathology, Diagnosis and SubDiag and label the table "Menus". I'll also call your main table "MedRec" and put the same three text fields in it with the same names.
Create a relationship linking MedRec and Menus by the Pathology fields (MedRec:: Pathology--=--Menus:: Pathology).
Click Menus to select it and click the "++" (two overlapping plus signs) to create a second occurrence of Menus.
Create a 2nd relationship: MedRec:: Diagnosis--=--Menus 2:: Diagnosis
Click OK to leave Manage | Database
Select Manage | Value Lists
Create a new value list "Pathology" and specify the Pathology field in Menus.
Create a new value list "DiagnosisByPath", specify the Diagnosis field in Menus, but also select the "include only related values starting from" option and select the MedRec table from the menu to the right of it.
Create a third value list "SubDiagByDiag", specify the SubDiag field in Menus 2, select the "include only related..." option and choose the MedRec table.
Populate your Menus table with at least a few test values. Every field of every record in this table should not be blank.
Set up your combo boxes for a layout that refers to MedRec, specifying these new value lists.
Enter browse mode and try them out.