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.

Old school — you could use dd:

dd if=A_FILE bs=1 skip=3

The input file is A_FILE, the block size is 1 character (byte), skip the first 3 'blocks' (bytes). (With some variants of dd such as GNU dd, you could use bs=1c here — and alternatives like bs=1k to read in blocks of 1 kilobyte in other circumstances. The dd on AIX does not support this, it seems; the BSD (macOS Sierra) variant doesn't support c but does support k, m, g, etc.)

There are other ways to achieve the same result, 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.