Is there a way to Add a 3rd XLR audio source to a prosumer camera ( EX1, XHA1, HVX)... All these cameras have 2 channel XLR, but lets say I want to add 2 Wireless lavs and still use an on camera boom. Is there a simple cable that can combine 2 channels into one ? ( both Lavs signals combined into same channel?)

I realize that this is not ideal. I realize ideally you would have a guy with a mixer, but I have been using a XL2 which has 4 channels and i want that ability on my other cameras...

Using a Y cable to combine 2 mics seems like a bad idea. I would look for a dual mic lav set-up that gives you one output or I'm sure there is some kind of mini camera mounted mixer you could get.

I think you answered your own question. You want/need a mixer. ENG-44 or SD 302 (3 channel) or SD442 (4 channel)... pretty much your best choices at this point in time. I am sure other adherents to various mixers I did not name will chime in and make their case for their favorites.. Those are my recommendations for you to choose from.

If you end up using a mixer you will want to be careful how you combine different mics to the same audio track. Mixing the 2 lavs to one track and the boom to the 2nd track would be my initial preference if the boom is being used all the time.

You really want a mixer for this, but to tell you the truth, mixing three down to two is almost always a bad idea. One option might be a dual receiver that will let you mix both wireless down to one channel right in the receiver. The AT1800 will do this.

One of these may be of some help:Juiced, Link, CX431, XLR, Audio, Adapter, Mixer, Preamp, (http://www.pinknoise-systems.co.uk/juiced-link-cx431-audio-adaptermixerpreamp-p-370.html?osCsid=287c308946182afd70693d9a3602ddb4)

FYI, there's an SD-302 for sale...

http://www.dvinfo.net/conf/private-classifieds/142440-sound-devices-302-field-mixer.html