How can I write these variables into one line of code in C#?

You can do your whole program in one line! Yes, that is right, one line!

Console.WriteLine(DateTime.Now.ToString("yyyy.MM.dd"));

You may notice that I did not use the same date format as you. That is because you should use the International Date Format as described in this W3C document. Every time you don't use it, somewhere a cute little animal dies.

You can do pretty much the same as in JavaScript. Try this:

Console.WriteLine(mon + "." + da + "." + yer);

Or you can use WriteLine as if it were a string.Format statement by doing:

Console.WriteLine("{0}.{1}.{2}", mon, da, yer);

which is equivalent to:

string.Format("{0}.{1}.{2}", mon, da, yer);

The number of parameters can be infinite, just make sure you correctly index those numbers (starting at 0).

Look into composite formatting:

Console.WriteLine("{0}.{1}.{2}", mon, da, yer);

You could also write (although it's not really recommended):

Console.WriteLine(mon + "." + da + "." + yer);

And, with the release of C# 6.0, you have string interpolation expressions:

Console.WriteLine($"{mon}.{da}.{yer}");  // note the $ prefix.