Old school — you could use dd:
dd if=A_FILE bs=1c skip=3
The input file is A_FILE, the block size is 1 character (byte), skip the first 3 'blocks' (bytes). There are other ways too:
sed '1s/^...//' A_FILE
This works if there are 3 or more characters on the first line.
tail -c +4 A_FILE
And you could use Perl, Python and so on too.