Your task still obscure and I think that you are confusing hex values and ASCII code.

please see if that is what you want:

you want to send the the characters ! , # and *. In hex they are, respectively, 0x20, 0x22, 0x2a. They will be sent over a serial line.

You think: how can I send these characters if they are over 15 (0x0f, in hex)? 0x0f is the largest value if you are using just 4 bits.

In fact you are probably using 8 bits, so you can send a value up to 0xFF (255 decimal)

hth... if not, please elaborate more your doubt.