No you don't. you need just one of them - they are all event providers - each utilizing a different underlying mechanism for message transport 

Simplest way to test is to edit a content in one instance and view it on another instance. the change should be seen (i.e. cache invalidation messages were sent through) 

@quan So applying Stefan Olsens solution and setting up the IP and ports is all I would have to do to ?

There are two settings you need to do

• setup Optimizely to use your (custom) event provider
• setup your custom event provider so it can send and receive messages over network

I'm not quite sure about (2), but if that's what in the documentation, yes. 

I ended up using Stefan Olsens solution with UdpMulticastEventProvider and configured the Host and Endpoints for each specific server (one authoring and two frontservers).

Then had to make sure that the port using UDP trafic was open between the three servers. (Rather OUT from Authoring and IN to frontservers).

After that it worked great.

Big shout-out for Stefan!