http://www.htc.com/us/faqs.aspx?p_id...&id=113506&f=t
Q. How do I make a three-way call?
1. Press the TALK/SEND button to open the Phone screen.
2. Make the call to the first participant. When the first call is connected, make a call to the second participant. The first call is put on hold when you dial the second call.
3. When the second call is connected, tap Flash to start the three-way call. The screen displays Call in progress.