Windows Forms

Yes, Regex is definitely the way to go here.

When you say there should not be more than 12 words in the TextBox, I assume you mean that there shouldn't be more than 12 characters but (presumably) at least 6:

using System.Text.RegularExpressions;

// ...

string r = @"^3(R|C)(OF|OH|OD)(B|\d)\d{1,7}$";
bool isValid = Regex.IsMatch(textBox1.Text, r);

Note that the above is case sensitive.

if you want it case insensitive, then change the regular expression to:

string r = @"^(?i)3(R|C)(OF|OH|OD)(B|\d)\d{1,7}$";