How to write if condition.?

Try something like:

if (condition 1)
    {}
else if (condition 2)
    {}
else
    {}

Advice! Take time to learn properly the language.
Here is links to references books on C and C++ by the authors of the languages. Note than C is the ancestor of C++, so knowing C is always useful with C++.
The C Programming Language - Wikipedia, the free encyclopedia[^]
https://hassanolity.files.wordpress.com/2013/11/the_c_programming_language_2.pdf[^]
http://www.ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf[^]

C++ Programing Language[^]

[UpDate]

Quote:

i already know the thing you provided but i want to write the case statement as below
or moreover in place of case 1 i want case if(conditional expression)

It does not exist in the language !

[UpDate]

Quote:

I want to optimize the below code using switch case statement.

i have already written this code.

C++

if (i % 3 == 0 && i % 5 == 0)
                {
                    Console.WriteLine("Both True");
                }
                if (i % 3 == 0)
                {
                    Console.WriteLine("Three True");
                }
                if (i % 5 == 0)
                {
                    Console.WriteLine("Five True");
                }

You can't ! it is already optimized !

Well that's meaningless (see switch (C# Reference)[^])

You could write

switch (expr)
{
case 0:
  if(i%3==0 && i%5==0)
  {
    //...
  }
  break;
case 1:
  //...  
}