Hello,
    Here's my setup, I replaced a MC3 with a CP3N (RS-232 and the three IR functions have been moved over from the MC3) but I also have a Crestron CLW-DIMEX-P controlled by the built-in MC3's RFGW-EX gateway to control the lights which aren't working now by Crestron.

1) How can I access the built-in gateway in the MC3 to control the lights?
2) How can I setup EISC correctly? (It didn't work for me, it looped the light control page on the panel.)
3) Can I use Cresnet?

Thank you for your future ideas.

I would make the MC3 a slave.
That way all of the IOs including the EX gateway will be available in the CP3.
I am embarrassed to say that I am not sure about the cresnet. I would assume that would need to be moved over to the CP3.
I dont know how the cresnet port behaves when the MC3 is set to slave. 

Your idea is the best, cresnet is about 38400 baus rate, super slow. You can also try the EISC but it will requiere having two running program and get things synchronized 

EISC for sure.  I'd put only my button presses and feedback to the CP3, and do any lighting logic local to the mc3. I'm not sure you can use an eslave device's gateway, I know you can't use it as a cresnet bridge.

I think the Cresnet port -IS- available on a 3-series processor in Slave mode.  Answer ID 5232 describes it, and I'm fairly certain I've done it before.