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.