There were some 3 deuce setups produced by people like Offenhauser, and others in the 60's and 70's. Ford also sold their own 3 carb setup. All of those are very hard to find now and I haven't seen one in years. I would guess they do turn up on Ebay occasionally, but I bet the price is high.
More recently, I think Barry Grant made a tripower setup for the sbf, but not sure about that. The problem with most of the manifolds out there is that the carbs are very crammed together due to the short area available between the back of the motor and the distributor, and most use Holley carbs with the air cleaner horn cut off flat to clear the next carb in line.
I'm a Ford guy, but have to say the ones made for sbf are not nearly as attractive as the ones for sbc engines, they just look squashed together or something. I saw a 3 carb setup for sale in a local speed shop one time about 20 years ago and passed on it for my own sbf in my 27 because it was built from modified Holleys and I just didn't like the way it looked. So I went with the current 2 x 4 setup I have been running on it for years.
But if your heart is set on having one, keep checking Ebay and also at very large swap meets some guys show up with very oddball intakes and they might have one of the early aftermarket ones. But be prepared to pay good money when you do find one. Good luck.
Don
Here, I looked on Ebay for you and found this one. It will go through the roof when it closes, I bet. I'm predicting $ 3000.
http://cgi.ebay.com/ebaymotors/RARE...tZVintageQ5fCarQ5fTruckQ5fPartsQ5fAccessories