During my trip to BVE, I had a look at the new Canon C100 to discover the handle with XLRs and built in mic was the same connection as used by the C300 that fits into hole no. 2.
Due to a stupid design flaw the C100 handle cable is about 70mm short when mounted onto a C300 as can be seen above.
If Canon could add the extra 70mm or a 3rd party produce a small extension cable…C300 owners could do away with the useless monitor on top of the C300, giving us a far neater XLR solution and have a built in mic for general usage.
Please note if Canon do produce this for the C300 can I get one for giving you the idea in the first place !