the 'Recreate initial objects' action
I didn't know it can recreate objects from other layouts, wow!
My top three:
1. "Find all references" for objects, variables, functions etc. Very handy!
2. Tab+click to select bottom object and "Show translucent inactive layers" checkbox in layout editor. I discovered these features just recently, and they make the job of editing big complex layouts so much easier!
3. I'd really like to see this idea implemented:
construct3.ideas.aha.io/ideas/C3-I-1419