Since Thomas has the same number of children as Peter and John have in total, we can conclude that Thomas has 3 children while Peter and John have 3 children in total. Since John has one child more than Mr. B, but can't have more than Thomas, we conclude that Peter is Mr. B and John has 2 children while Peter has only one child. Since Mr. C has the same number of sons as Peter, who was only one child, we know that Mr. C has at most one son. If Thomas was Mr. C, he had at most one son, thus at least 2 daughters, which can't be true. Thus, John is Mr. C and Thomas is Mr. A. Since we now know that John and Peter have the same number of sons, while John has 2 and Peter has 1 children, we conclude that both have one son and John has the only daughter.
Thus, the name of Mr. A is Thomas and he has 3 sons.