Adobe introduced a new feature in CC called Extract Assets. I haven't played with it yet, but it looks like it has options for exporting at multiple resolutions.
In the past I've used an app called Slicy (Mac only I'm afraid) which let's you output 1x and 2x resolutions. I still had to create a 1.5x version in the same file (which Slicy would output as a 3x version), so there was some duplication, but better than doing it all manually.
If you're doing 9-patch you'll find the guides are a bit trickier, since they shave to be 1px wide regardless of the resolution. I'd set them to be 1px at the lowest resolution, (At the time I was only going as low as MDPI). My guides would then be by thicker at the higher resolutions (2px at HDPI etc.) I'd pass these PNGs through a bash script to trim off the extra pixels using the
mogrify shave command. Finally I'd automate it using Hazel (Mac app again) which would watch my folder for changes, and apply the script to any new PNGs. It shouldn't be too hard to find an equivilent solution for watching folders on other platforms.