3-10 was a nightmare for me, too, until I read about only using two stations and the platform. I did that and was onto level 4 in two more tries. I have been using Scarlett since the beginning of the game so I don't think switching would have made a difference. Since starting level 4 I have only used two stations (one for dogs and one for cats so I get the switching/flag bonuses) and the platform to increase hearts in the waiting line. That way, you can take more time. Also, you can put treats on the seats of the accessories stations so that when you put the animal at that station they will want to go in the playpen. Make sure to buy two playpens, though. One for cats and one for dogs.