After playing I've found it will automatically deal the next row of 3 without you having to do anything. That way there is always a possible set on the screen. Definitely a bit uncomfortable wondering if you are looking for a set that is not possible.
Sometimes no player finds a set. In that case in real life it often helps to rearrange the cards. Maybe this would be a good feature
The computer takes care of that automatically, and deals the extra 3 cards. It's covered in the help page. :)