I was talking with a fellow brewer who has been brewing for many years and found out that he does not put any liquid in his 3 piece airlocks. He claims that next to nothing can get in even without the water that there so he just stopped, and he claims the water is just to see the gas coming to to show that it is fermenting.
Is liquid actually required in the 3 piece airlocks?
note: his beers always taste great and never seem to get infected.
Look up Pasteur's goose-neck flask experiment.
Passive environmental dust/contaminants wouldn't be able to get through a 3-piece airlock, no.
More active actors (fruit-flies, spiders, ants, &c.) would; liquid would be an effective deterrent to them.
As mentioned, gas exchange would be possible w/o liquid. During active fermentation, there's enough pressure from the produced CO₂ to prevent this very effectively, but once fermentation subsides, the air will begin to mix/normalize with the CO₂ in the headspace. The relatively small area of the airlock stem would limit this pretty effectively, though, I would think.
Liquid in the airlock is required if you want an airtight seal.
If you don't care about having an airtight seal, then you don't need liquid. By not having an airtight seal, you risk air getting into your fermentor and bring along things which you may not like (i.e. oxygen, bacteria, etc.). Now with the 3 piece, the risk is less because you at least have a piece of plastic resting on the tube which actually enters the carboy. But, alas, it is not an airtight seal.
