Does anybody know any software with GUI interface for sending USSD commands to 3G Modems?
I have found only this but it uses xml file and you need to store commands in xml file before sending.
Thank you in advance.
It is Modem Manager. It can be installed by
sudo apt-get install modem-manager-gui
Well, if you want to check your balance or top up on a SIM-Card, there is the Prepaid-manager-applet. It has a nice graphical interface and is in the official repositories: 
If your provider is not already listed, you can add the USSD-number to '/usr/share/mobile-broadband-provider-info/serviceproviders.xml'. Find the section of your country and provider and add
<balance-check>
<ussd>*102#</ussd>
</balance-check>
whereas *102# has to be replaced by your providers USSD-code.
If you also want to top up your phone with this applet, you also need to add
<balance-top-up>
<ussd replacement="CODE">*130*CODE#</ussd>
</balance-top-up>
And secondly you can also send USSD commands via gammu. Install wammu for a graphical interface. The configuration of gammu will be more complicated but it offers a lot more functionality -- you can send USSD commands of any kind, not just for balance-check.