All information suggests it but still it’s not 100% sure.
System: Android 6.0.1.;
Screen: 4,6″ 1080p;
SoC: Snapdragon 820;
RAM: 6GB;
Storage: 64GB;
Cameras: 15/7MP.
Rumors about OnePlus 3 in Mini version fflow for some time in internet but we had none, even least, evidence to confirm them. Now we have a makeshift of it, but still no confiration from trusted source.

Source: GSMArena via GFXBench