Yea, 3 piece airlocks will cause suckback when you apply negative pressure (cold crash- the air inside the carboy contracts as it cools). 3piece airlocks or just using an aluminum foil cap will prevent that issue, but it will continue to be a problem as long as you use 3piece airlocks. Use vodka or sanitizer in your airlock if you're concerned and don't want to buy new airlocks.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
twitter.com/2kidsbrewing .. facebook.com/2kidsbrewing .. 2kidsbrewing.com