dairy, anything with added sugar or added salt or added fat are very unhealthy for rats.
A piece of ripe banana is a good treat, and vegs like peas and carrots are good as rats need a variety of vegs every day (such as baby kale, broccoli, cooked squash, cooked sweet potato, mixed greens, cauliflower, a couple of cooked chick peas, a couple of cooked kidney beans, etc). Other good treats are berries, other types of fruit, cooked whole grains, etc There is a forbidden foods list.
Is the bath tub their play area? Because I don't understand why you are putting them in what would normally be a scary place, one at a time.
When I feed my rats, or give them a healthy treat I call out the word treat ..... so they will all learn to come to me when I say treat. It is good to have a word that will cause them all to come to you.
When they are out in the play area you can say a name and then give that rat a treat. That way each will learn to associate their name with a treat and eventually will come to you when they hear their name.
Treats I would suggest as rewards are one organic kumat puff, or an organic pumpkin seed. Because these are small, something they will like, are not too fattening in moderation, and can be eaten in seconds.
Perhaps start with trust training and getting them used to you. Put on a shirt with a loose fitting shirt over it. Then put all 3 rats on you between the two shirt and sit or lay down and the rats can get used to you and your smell and feel safe. Do this for at least 20 minutes at a time.... and if they decide to climb out and wander around exploring or climb on you that is ok.
Your rats are active and curious ...... they need to run and climb and explore and interact with you in a safe area with toys and lots of interesting things for them to explore