THIS TRICK WORKS FOR THOSE E303 MODEMS MADE WITH FIRMWARE UPDATE BELOW VERSION 21.157.31.00.850
Note:
If you are sure you don't know how to do it perfectly follow this link to Unlock it with DC-Unlocker.

Now Make sure your Modem Firmware version is below the above firmware version.(very important!)

FOLLOW METHODS BELOW TO PERMANENTLY unlock HUAWEI E303
Unlock modems by updating its firmware.
1. Download Universal Master Code
Calculator so that we get the FLASH CODE
.
Download from Here

2. Put those IMEI codes into "Universal
Master Code Calculator" that you have
downloaded before and get the FLASH
CODES that we are going to use them as a
PASSWORD in the next step.

3. Download HUAWEI E303 firmware
update the address here:-
Huawei Modem FIrmware from here

4. Unzip the file and make sure the modem is
connected and it has been installed with the
SIM card inside it.

5. Quit the dashboard (Interface)if its open and run the
update, wait and when it ask you for the
PASSWORD input the FLASH CODE you
got from Universal Mastercode Calculator..
Wait for about 3 min and your modem will be
unlocked PERMANENTLY.

If in any case your modem E303 firmware version is above stated, i advise you follow this link
It is now at your disposal free of charge