Visual Studio – How can I generate a converter program from Decimal to Xs3? c #

I'm new to this area, but my question is this.

I have this code:

public long xs3 (long a)
{
String cad = Convert.ToString (a);
long sum = 0;
foreach (char c in cad)
{
sum = long.Parse (c.ToString ()) + 3;
string binar = Convert.ToString (sum 2);
long i = 0;
if (! Int64.TryParse (binar, out i))
{
i = -1;
}
return i;
}
Repayment amount;
}

To remove excess 3, you must add +3 for each of the separately entered numbers.
Example:

long n = "126"; // Already added, they would have to give "459"

And then pass each number to binary

// I would have to give 0001 0010 0110 

My question is … how do I add each number entered in the string separately because my code only adds the first data entered and only if it is one, if I enter more, will the program take it as if I only had it introduced one
Example:

When issuing a single number entered: 6 = 9 in binary -> 1001
When outputting two numbers to: 11 = 44 in binary format should be 0100 0100
However, only 0100 appears on the screen, thus losing a loss in the existential abyss. 

So my question is how do you continue to add data and continue printing so that all the data is displayed instead of just one?