How to read tcvn3 character in any process?

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C#3.5

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Dear all
Can some one tell me how to read TCVN3 character in process (I control) by C# programming language.
I See C# only Support ASCII and Unicode? is it right?
I Had use below code and received Character same here
- Use ASCII received: Tr?i Cung, T?
- Use Unicode received: T#?i Cu#g, T?#
The problem is how to Read Full Character TCVN3 in process!
Thanks a lots for help

What I have tried:

My Code Read memeory Process: use ReadProcessMemory (API)

public class clsMemoryEditor
{     
        [DllImport("Kernel32.dll")]
        static extern bool ReadProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, byte[] lpBuffer, UInt32 nSize, ref UInt32 lpNumberOfBytesRead);
        IntPtr hand;
        public clsMemoryEditor(IntPtr handle)
        {
            hand = handle;
        }
        public byte[] Read(int Address, int length)
        {
            byte[] ret = new byte[length];
            uint o = 0;
            ReadProcessMemory(hand, (IntPtr)Address, ret, (UInt32)ret.Length, ref o);
            return ret;
        }
        public int ReadInt32(int Address)
        {
            return BitConverter.ToInt32(Read(Address, 4), 0);
        }       
        public string ReadString(int Address, int length, bool isUnicode)
        {
            if (isUnicode)
            {
                UnicodeEncoding enc = new UnicodeEncoding();
                return enc.GetString(Read(Address, length));
            }
            else
            {  
                ASCIIEncoding enc = new ASCIIEncoding();
                return enc.GetString(Read(Address, length));
            }
        }
}