Sorry to hear you are having issues.
It is not a good sing that you odnt get a picture form the seocnd Mini DP Output. What R9 270X do you have btw, is it a MSI R9 270X ITX version ?
It can happen that Catalyst will automatically place the resolution in some Ultra High Setting because it detects 2 screens.
We did not complete testing with the Beta drivers, so i will advise you to use the 15.7.1 WHQL.
Connect the MST Hub to the DP Output you now works and only attache 1 screen to the MST Hub and boot up the system. Check in Windows if all is ok. Only then connect the second screen, check in windows if all is ok, sometimes you will need to activate it first and most important check the resolution and frequency. Only than go for the 3rd screen, repeat the same windows check. When done go tot the Catalyst and setup your Eyefinity group etc. Don’t forget to make a preset (left top) when your done with the settings, very handy to go back to your preset-settings when needed with just 1 click