Just buy polyvoysol sp? with NO Iron for children... Works great for goslings too. Plus there I know there are vitamins out their for waterfowl. I have never had to use them..
But have heard they work great to kick start runts is what I call them.
These also may be females because females are smaller than ganders. But my gosh what a difference in sizes in those goslings. I would say the two are ganders and the little one female..
You could spoil them even more by feeding them some romaine lettuce by their self so they get the extra greens to help them as well. Some babies are runts and just need extra care and food to help them develop better.
You are feeding a good diet to them it is just the little runt needs some special attention.
Offer some cut up green or red grapes, watermelon, cut up apples, cut tomatoes, peas, some corn not a lot, as I said romaine lettuce, kale, broccoli, anything you can think of give it a try and see if they like it. All you can do is offer it if they like it they will eat it. Cucumbers, zucchini, yellow squash. Now they are getting older try some oats. There are many different things that geese will eat. Pumpkin while young or they will not try it as an adult.