Xcode 6 asset catalog automatically create images for smaller scales
I ended up making a small mac app that takes care of the dirty work. You can drag and drop one or multiple .jpg or .png files into the app's main view; it will detect the input image's scale and create the smaller scaled images. With this app I only need to create a single, 3x image and drag it into the app to create the two smaller images. It saves so much work!
The source can be found on my GitHub page, simply download it and run it on your mac! https://github.com/JonasGessner/ImageReducer
There is another way to solve this problem.
Use Automator to create Mac OS X service which create @3x, @2x and normal image on base of 1 image in @3x resolution automatically. It use just 3 methods: duplicate images, scale down, and rename.
Ready Mac OS X service made in Automator you can find here:
late to the party but there is one tool which make all assets @1x, @2x, @3x and icons for you in just one click. And also export to XCAssets file.
You can find it here: https://github.com/angelvasa/AVXCAssets-Generator
Hope this will help someone