I was successful connecting a sound module to my T2 when I had it. Now is a good time to study how all the MIDI messages work, then you can even put together your own custom MIDI setup for your T3.
It will work, however, you will run into the same darn issue I did...Yamaha's MIDI implemtation on the arrangers is not very good for what you want to do.
What I wanted is when I select a songbook entry, or a performance (registration) the new sound I setup on the external sound module would recv the correct MIDI bank/program change messags to select the sounds for each channel (upper 1,2,3, lower ect.) Unless The T3 is different, a user voice sends MIDI data based on the default for that sound in the T3 (the user slot it occupys)...and as such you can not program the MIDI messages you actually want to be sent out.
Not good.
When I called Yamaha and presented this issue to them...they promply told me I don't need to use a sound module! In other words, they don't support that level of implemtation beacuse this is a home keyboard.
I sold the T2 and now have a Korg PA2XPRO which supports all the PRO level MIDI stuff.
Each user sound can be programmed very easily to send out any bank/program change you need. So, when you use that sound in a performance, songbook etc..it works fine.
You will be able to use a sound module (many here have), just that you will be limited as to how it works. Also, not many of the controllers on the T3 (assuming it's like the T2) will send out MIDI data to a module.
If you setup a combi on the external module that has everything you need and then asign to the same MIDI message that the T3 sends by default (based on the user sound slot) then it will call it up.
Like I said, I made it work, but it was very poor.
Lee S.
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Lee S.