You should convert the Login control
to template (Use smart tag at design time) and then place the RadCaptcha control inside the Login's control. Make sure the Captcha has the same ValidationGroup set as the Login Button.
Initially the RadCaptcha should have Visible="false" and Enabled="false".
You should handle the Login.LoginError event
to count the times a user has failed to log in. If there are more than 3 times, the RadCaptcha's Visible and Enabled properties are set to true. Use Login.FindControl("RadCaptcha1") as RadCaptcha
to find the Captcha control placed inside the Login control.
You should also handle the Login.LoggingIn event
to test for the validity of RadCaptcha (RadCaptcha.IsValid). If the Captcha is not valid you should cancel the event.
I hope this helps.
the Telerik team