Code:
string txtBox = [RichTextBoxName].Text;
string txtBox1 = [RichTextBoxName1].Text;
string txtBox2 = [RichTextBoxName2].Text;
StreamWriter file = new StreamWriter([Pfad TextDokument]);
file.WriteLine(txtBox);
file.WriteLine(txtBox1);
file.WriteLine(txtBox2);
file.Close();
Hast du etwa so was gemeint?