There's a lot of yap-yap on this forum about filling a 3piece airlock with miscellaneous *safe* liquids in case of a suck-back.
This post is to show how one can fill a 3-piece airlock that will only allow air (and not liquid!) to be sucked-back in the event that there's a pressure differential.
Soak in a a bowl of sanitizer at least 4" deep:
Assemble the 3 piece while still covered in the sanitizing solution:
Up-right the now-assembled airlock, and pull it out of the sanitizing solution. The negative pressure will suck liquid out of the airlock, until it starts sucking air. At this point, it's not possible anymore for liquid to be sucked back.
It's a bit hard to take a picture of (because it happens under 2 seconds), but in this pic the water is draining from the bottom of the airlock:
And the result is a perfectly filled airlock with no possibility of liquid suck back:
Give a small shake to the airlock to release any liquid trapped in the stem.