How to convert integer to binary string in C#?

Simple soution:

IntToBinValue = Convert.ToString(6, 2);

Almost all computers today use two's complement representation internally, so if you do a straightforward conversion like this, you'll get the two's complement string:

public string Convert(int x) {
  char[] bits = new char[32];
  int i = 0;

  while (x != 0) {
    bits[i++] = (x & 1) == 1 ? '1' : '0';
    x >>= 1;
  }

  Array.Reverse(bits, 0, i);
  return new string(bits);
}

That's your basis for the remaining two conversions. For sign-magnitude, simply extract the sign beforehand and convert the absolute value:

byte sign;
if (x < 0) {
  sign = '1';
  x = -x;
} else {
  sign = '0';
}
string magnitude = Convert(x);

For one's complement, subtract one if the number is negative:

if (x < 0)
  x--;
string onec = Convert(x);