words.cs

// 1. use List.Add, hope that .ToArray() works
List<string> temp = new List<string>();
temp.Add(desired_username_tb.Text);
temp.Add(new_password_tb.Text);
temp.Add(Email_tb.Text);
DB.login_DB.Add(temp.ToArray());

// 2. Use an array of strings
string[] temp = new string[3];

temp[0] = (desired_username_tb.Text);
temp[1] = (new_password_tb.Text);
temp[2] = (Email_tb.Text);
DB.login_db.Add(temp);