3 indicates the column width occupied by the input. Although the former is decorated with three characters, it can only store one character for ch (I don't know why you define it, but only one character for char).

The following 3d indicates that the input is an integer and the input is 3 columns wide, so 12345 123 is taken as the input value. (Even if it is defined as char, 123 can be saved without overflow. )