Hey @Sradha, Yes it is possible in TestNG.
All you need to do is create a XML file and list all the different class you want to test at once.
Here is the syntax:
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name = "TestNG">
<test = "Testing TestNG">
<classes>
<class name = "testNG.class1"/>
<class name = "testNG.class2"/>
<class name = "testNG.class3"/>
</classes>
</test>
</suite>
This way you can run the test cases in each class at once.