I'm glad to hear these are not chicks you are trying to wean.
It's actually easier to wean a bird onto new foods alongside birds that are already eating these new foods. It's kind of like "Monkey see, Monkey do". Your bird will see that the others are enjoying this food, and they are more likely to go for it themselves.
I would suggest putting the bird back with the other two, and just be patient. I'm sure he'll come around eventually.