C if statements skipped over - c

Question

I'm currently learning C and am attempting to do the reverse of what some courses are asking for. miniMasterMind is an assignment I found where the user guesses numbers randomly generated by the computer. I'm attempting to make a simple flip on it, where the user tells the computer whether or not its guesses are correct for a user generated 3 digit number.
I have what I think is a fully working program, except my 3 if statements asking for user input sometimes do not work. I can't see any reason for it, but after compiling I often find one or two of the if statements to just skip over the user input. I put in system("pause")'s after each step to make it easier to see.
Each turn in the game, a different set of if statements seems to break. Why is this happening?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
// Initialize variables
int UCMain = 0;
int CG1 = -1, CG2 = -1, CG3 = -1;
int win1 = -2, win2 = -3, win3 = -4;
char check1 = 'A', check2 = 'B', check3 = 'C';
int turnCount = 0;
// Print out start screen
printf("Welcome to masterMind reversed! Let's see how this works out!\n\n");
// Accept user input
printf("Type in a three digit number for the computer to guess.\n");
scanf_s("%d", &UCMain, 3);
const int UC1 = (UCMain / 100) % 10;
const int UC2 = (UCMain / 10) % 10;
const int UC3 = UCMain % 10;
printf("\nTest print, UC1: %d UC2: %d UC3: %d\n", UC1, UC2, UC3);
system("Pause");
// Start game loop
while (turnCount < 10)
{
// Random number gen
srand((int)time(0));
// 1st number
if (win1 == UC1)
{
CG1 = win1;
}
else if (win1 != UC1)
{
CG1 = rand() % 10;
}
// 2nd number
if (win2 == UC2)
{
CG2 = win2;
}
else if (win2 != UC2)
{
CG2 = rand() % 10;
if (CG2 == CG1)
{
CG2 = rand() % 10;
} // End unique check
}
//3rd number
if (win3 == UC3)
{
CG3 = win3;
}
else if (win3 != UC3)
{
CG3 = rand() % 10;
if (CG3 == CG2 || CG3 == CG1)
{
CG3 = rand() % 10;
} // End unique check
}
// End random number generation
printf("The computer guesses: %d%d%d\n", CG1, CG2, CG3);
system("Pause");
// Check if numbers are correct
if (win1 != UC1)
{
printf("Is the first number correct? Y/N\n");
scanf_s("%c", &check1, 1);
if (check1 == 'Y')
{
win1 = UC1;
} //
}// End 1st check
system("pause");
if (win2 != UC2)
{
printf("Is the second number correct? Y/N\n");
scanf_s("%c", &check2, 1);
if (check2 == 'Y')
{
win2 = UC2;
} //
}// End second check
system("pause");
if (win3 != UC3)
{
printf("Is the third number correct? Y/N\n");
scanf_s("%c", &check3, 1);
if (check3 == 'Y')
{
win3 = UC3;
} //
}// End third check
system("pause");
// Check if game is over
if (win1 == UC1 && win2 == UC2 && win3 == UC3)
{
printf("The computer wins!");
}
turnCount++;
} // End while
// Win/lose state
if (turnCount == 10)
{
printf("The computer loses!");
}
}

Answer 1

it is not really skipped:
it is taking a character the newline character: '\n'
Using a scanf with a space before it:
scanf(" %c", &b); // this one will work instead
Will tell the scanf that any white space characters (including the newline '\n') left on stdin should be ignored.
please read more about scanf here

Answer 2

scanf() is reading all user input, including line feed and carriage return characters. If the user types "1" and presses enter, you'll actually get 2 (or 3, depending on the platform) input characters, one for the digit and one or two for the newline character(s). This will stimulate 2 or 3 iterations of your loop when you only intended one.