Building dependent comboboxes on an existing userform

I have nearly completed the work but now have been asked to "attempt" to build dependent comboboxes

Ideally the end user will select a name from the REPRESENTATIVE field and it will auto select the SUPERVISOR and SITE LOCATION but leave the ability to make alterations (in the event that the entry needs to be changed)

I have been able to limit the options using data validation but unsure how to write the code in VBA. I have attached a test database/userform and an example of how the information should be linked (see Sheet3)

Note- this should be able to function for 700+ representatives, 30+supervisors, and 6 site locations.

any assistance so I can finally put this project to bed would be great.

See the attached, for an example.
Notice on the UserForm initialize how Rep ComboBox gets Populated
Notice the Combobox3_Change code

Select from the Rep combobox and the textboxes populate