How to block internet access of all apps (except browser) when using 3g usb modem?

Is there any app that helps me to block all apps for accessing the internet when using 3g modem?

Little Snitch is like a Ferrari if you can afford it.

There is a now discontinued little app that does the same for 10.6 and before Yosemite.

TCPBlock:

Allows you to select who is to be blocked and who not.

Yes, (assuming you are using Mac OS X), but you're not going to like the answer: pfctl. (Prior to OS X 10.7 Lion, the firewall was ipfw.)

Also, alternatively, you can use the GUI to configure pfctl (or ipfw).

Also, OS X includes an application firewall :

Allowing specific applications

To allow a specific app to receive incoming connections, add it using Firewall Options:

• Open System Preferences.
• Click the Security or Security & Privacy icon.
• Select the Firewall tab.
• Click the lock icon in the preference pane, then enter an administrator name and password.
• Click the Firewall Options button
• Click the Add Application (+) button.
• Select the app you want to allow incoming connection privileges for.
• Click Add.
• Click OK.
• You can also remove any apps listed here that you no longer want to allow by clicking the Remove App (-) button. 

There is also a third party application that is very popular called Little Snitch that will do what you ask.

Although I wouldn't recommend it for your purposes unless you were using it to learn about it for its own sake, OS X 10.6 Snow Leopard and above includes a sandbox facility (be sure to read the man pages listed under "SEE ALSO.")