Let me get this right. You have three 1920×1080 screens and one that has 2560×1440 resolution and you want all of them to run at their native resolutions with the R7 260X.
Can you tell us what MST Hub are you using from us?
If you are using 4 displays, you can use the 260X native 4 ports to get 4 screens simultaneously, the only requirement is to have an active adapter on the DisplayPort output in case you do not have a DisplayPort monitor.
To setup an Eyefnity configuration you need to go to the Catalyst Control Center and under AMD Eyefinity Multi-Display you can configure the displays based on your physical positioning of your displays.
If your objective is to create a mixed configuration setup please read this article, I think it might be useful for you. You can find the link here
There are some examples of the setup
Now, in order to create a 3 screen setup in SLS (Single Large Surface) or Eyefinity 3 just pick the 3x 1080p displays only. Once you do that as explained above, create the group and then save the configuration. You can save the configuration under CCC in the same menu. Also you can adjust he bezels, this is important so you do not have crooked images from one monitor to the next.
After you create this group, under Windows Display Properties you will see two displays only, one with the 2560×1440 and the other as a 5760×1080 resolutions, drag the displays as you wish based on your physical position.
The guys at WSGF made a similar configuration as you want here. The main difference is that they are using a MST Hub 1-4 to drive all 4 displays. As they are using 3x 1920x1080p and one 2560x1080p screens, they just came short to the full bandwidth on a single DP 1.2 video output. In this case if they are using a 2560x1440p screen it will not be possible as they will exceed the bandwidth.
If you have more questions please let us know and welcome to insights