number increment depending on a combo box entry

hi,

i am new to access. currently, i'm working with multiple tables and forms.

in one form,
i have text field: txtClientID; Data type: Text, which i use as primary
key 
to number my entries; input mask "L000".
i also have a combo box, cboClientStatus, which identifies the 
status/category of my clients, e.g. Hospital, Clinic and Other.

now, i need ur help to set the primary key to increase by 1 depending on
the 
status/category of my clients, like:
H001, H002, H003, ... (for Hospitals)
C001, 002, C003, ... (for Clinics)
O001, O002, O003, ... (for all other than Hospitals and Clinics)

i want the number to increase automatically when i choose the status of my

clients. i know i need a vb for this, probably at cboClientStatus in After

Update event. can u guys pls help me with this.

thanx a bunch.