I would not say no choice, although clearly something like MainLobby (or some other package for integration such as CQC) can be configured to maximize the use of that type of equipment, whereas MCE is not in the same league.
That being said I'll take a stab at what it might look like, in case you are hooked on the interface and setup (as if you spend the additional effort/cost on integrating this further you will have more options from your setup in addition to being able to tailor it further not to mention control it fully).
1. You have three screens, which I assume would be in three rooms. There are two options here:
a. A MCE machine in each room - This is more expensive as each MCE machine is a computer. However, this is not required. See the next option, although you have 2 MCE machines already so this might be easier in your case. It also might give you larger storage options.
b. A main MCE machine and extenders in the other rooms using XBox or XBox 360's. I believe I am about to get one of the xbox 360's to test as an extender as I need an extender and I am trying to verify the concept as it would be ideal in a situation I was asked to help out with, which would utilize these extenders.
2. You have zoned multi-room music. A few options here:
a. Hook the output from the main MCE machine to the CAV6.6. This would let your various zones listen to the output of the main machine (if I remember the options for the CAV6.6 correctly).
b. Better: if you can hook the output from each of the mce machines/extender to the CAV6.6 as zone inputs (again taking liberty on incomplete knowledge of the CAV6.6) that might be a decent setup. There could also be some holes in this as well.
Note that something other than MCE such as MainLobby or the like could be a little more tightly integrated. This essentially just pipes an output (or more than one) into the CAV6.6 (and assumes you have existing control of the system). A more integrated solution could control the system more fully and offer more options.
3. The Audio Authority component in/cat 5 out, if that is the component video distribution setup theyhave, you might be able to use an ATI based card (I have not tried this) which has a component output dongle, which could feed into that system and be piped to the other screens. I've used VGA/DVI for my outputs but I would presume that component would look pretty good, although I have not tried this approach (but I considered it).
This might imply that you are sharing the system between rooms (which would mean they are all watching the same feed if only one is being distributed). Some form of IR distribution or equivalent would be required in this case (to control the system, select shows, etc).
Now you could also (depending on viewing considerations) put one MCE machine in the two primary viewing locations and then 'copy' the secondary one to the third screen (if both will not likely be used at the same time), again noting IR control issues at least.
Gyration 100' keyboard/mice combo's might be a good addition in this situation if you plan on using the keyboard/mouse.
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The above items try to cover what you are trying to do, and are fairly straightforward (especially if you drop component video distribution). It might be a good starting point to start to form opinions on what you 'really' want later.
You could then take the next step and go after the tighter integration that these other packages provide (with a little more complexity and setup to get it started). There are some very nice things you can, so you should take a look at what is available.
EDIT: Another option is to keep the MCE for primary viewing but use the integration packages to tailor the multi-room audio specifically. This might use touch screens or the like to select music that goes into the CAV6.6 and allows you to control that system.
You might want to think about and comment on your reasons for using MCE (don't get me wrong, I like it myself, but I am also working on how to get to the next level myself). Is it you really are hooked on the interface, or it is what you are familiar with? Or perhaps even, just the fact that you have two of thes machines already and want to deploy them...
