It won't.
The IX5000 is a 3 codec system, it will only send three images when calling into an MCU that supports multi codec rooms like a telepresence server/Webex, or another like (3 codec) system (another IX, CTS3000/3010, or TX9000). A Room Kit Pro is one codec.
If your IX is dialed into a Webex meeting room and you are allowing Multistream, you could have live video on two of your three screens, but if you thought a RK Pro with three monitors would behave like an IX5000 you were mistaken.