Write a program in C++ that prompts the user to input three characters (A-Z and a-z), then prints the characters in alphabetical order and meets the following requirements:

The program reads one input character at a time.

The program checks validity of input character right after it is entered. If valid, the program outputs confirmation message: You have entered X, where X is the input character, and continues execution. If not, the program outputs an error message: You have entered an invalid character. and stops execution.

When outputting the final outcome in alphabetical order, upper-case characters come first and then followed by lower-case characters.