iPhone6 Plus 2208x1242px | 736x414pt | 1920x1080px | 1pt = 3px
iPhone6 1334x750px | 667x375pt | 1334x750px | 1pt = 2 px
iPhone4s 960x640px | 480x320px | 960x640px | 1pt = 2 px
Here for details
Firstly, this is the size of the resources, the second is the physical point , the third is the screen size, iPhone6 plus is slightly connected, because its screen size is not equal to the size of the resources, and each resource will reduce the time by 1.15 times.
For your @ 3x question, the physical point of each device is used. therefore iPhone6 @ 3x should be 736x414px X 3 = 2208x1242px
Windsooon
source share