I think Sreeram and Rakesh have stronger, more mature voice qualities. But Rakesh lacks in the versatility department. And Shivam has that X-factor and youthfulness in his singing. He's a complete package, and not just a good singer.
But if we were to take everything else aside (X-factor, versatility, etc) and only focus on singing, I think Sreeram and Rakesh are technically better singers than Shivam.
I can see Rakesh getting a playback song right now, but I cannot see the same for Shivam.