The 3rd hole IS a breather, but I have yet to work out quite what for! The correct FZD inlet trumpets (with gauze) are still available on e-bay---£18-£21.
Alternatively, you could do what I have done when I wanted to fit a longer trumpet, which was to make a 10mm alloy spacer, drilled to fit the carb with countersunk allen screws and then drilled to accept a Weber/Dellorto bolt-on trumpet. The trumpet has to be turned about 45 degrees from the horizontal in order that you can fit the trumpet to the adaptor. When it is all lined up, you drill the 3rd hole in the trumpet lip and the spacer. Finally you carefully file the spacer and the trumpet lip just enough that the pump-jet can be removed without taking off the spacer OR the trumpet---simples.
On ALL FZD fitment to the 500/126 engines the Emulsion tube should be a 6747-6. These are difficult to find in the UK (there is a supplier in Italy who can supply them) but a 6747-1 can be modified to match a 6747-6 by the simple expediant of carefully soldering up the top 2 holes (1 per side)---it will then match a 6747-6. After soldering, make sure that you still have clear passage down the centre of the emulsion tune.

Thanks for the response. It does seem rather small to be a crankcase breather input.

SuperTopo said:

Thanks for the response. It does seem rather small to be a crankcase breather input.

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I stripped one down about a week ago and I remember thinking “ah so that is what that hole does”. Unfortunately I now can’t remember what it did so showing my age.