Insert ID Column value into table depending upon input filename

Hi Experts,

I am loading files with same format  from two different locations in to one database. First, my SSIS connects to location 1. 

Lets Say 
Location1 : C:\LoadFiles\Import
Location2 : D:\LoadFiles\Import

Location one has three different set files 
First Set: AP_1.txt,  AP_2.txt,   AP_3.txt, 
Second Set: VD_1.txt, VD_2.txt, VD_3.txt, 
Third Set: BK_1.txt, BK_2.txt, BK_3.txt ,

This SSIS set to run every 3 hours, if it finds files  of any set load them. While loading it has to insert into a derivedcolumn called CustID. If the file name Starts with AP_ , custiD values should be as 101
AP_1 goes to Table1
AP_2 goes to Table2
AP_3 goes to Table3

If the file name Starts with VD_ , custiD values should be as 201
If the file name Starts with BK_ , custiD values should be as 301

after processing all these files in first location, the SSIS looks for files in second location and does the same? 

--How to achieve CustID depending upon file name ?

please Help