What about just a generic achievement for building all three canals? CanAll?
I don't play as Venice (nor in Ironman), but in my very first Poland game, I built both the Suez and Panama canals as Commonwealth. I could have afforded the Kiel Canal in terms of both cost and diplomatic points, however Holstein was owned by my Danish ally, and they were too large to diplo-vassalize. Point being, it should be possible to build all three canals if you chose a strong starting nation or are a really good player (I'm honestly quite mediocre, which is why I always play mid-sized European nations).