python script to compare binary files in 2 different folder by ignoring first few byte

I'm new in python but I want to compare 2 folders with the same file structure for different file content.
Example:
Dir1 content: a.a b.b c.c
Dir2 content: a.a b.b c.c
a.a content in Dir1: 123asdfghjkl
a.a content in Dir2: 213asdfghjkl

So I want to compare in those 2 Dirs the files with the same name 1 by 1 by skipping the first 3 character and display the differing files.

I have checked the filecmp.dircmp() function but could you help me to extend the below script to skip the checking of first 3 byte during file comparation?

    #!/usr/bin/env python
    import filecmp
    import os
    directory1 = os.path.join("./","Dir1")
    directory2 = os.path.join("./","Dir2")
    cmp = filecmp.dircmp(directory1,directory2)
    print cmp.report()