Hello @Sradha,
You can create a TestNG suite to do that. This is also done in a similar way as used to run multiple class. All you need to do is create a XML file and then mention all the classes that you want to run, mention the group that you want to test. That's all. Here is an example for the same:-
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name = "TestNG">
<test name = "Testing TestNG">
<groups>
<run>
<include name = "A"/>
<include name = "C"/>
</run>
</groups>
<classes>
<class name = "testNG.Test1"/>
<class name = "testNG.Test2"/>
<class name = "testNG.Test3"/>
</classes>
</test>
</suite>
Hope this helps.