In case anyone is interested, there is a Healey parts lot on ebay that looks like it includes one intake manifold for a tri-carb. I would think these are fairly hard to come by.
https://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7903214364
https://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7903214364