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.