How to get custom field from BuddyPress Extended Profile

It’s probably due to the context of your code – which you didn’t explain.

Try this in your-theme/functions.php

function hisgen_check() {
   echo 'field: ' . xprofile_get_field_data( 3, bp_loggedin_user_id(), 'comma' );
}
add_action( 'bp_ready', 'hisgen_check' );