Why function hooked using object are executing at all time?

That happens, because you run the function in add_shortcode(). To send is as callback, use:

add_shortcode('examp_code', array ( $obj, 'ex_funct' ) );

Now you have still a problem: you create output in your shortcode callback. Never do that, you have to return a string.

Change it to:

function ex_funct(){
      return '<h3>Hello</h3>';
}