Can I run iPhone apps on iPad Mini 3 using iPhone 6 compatibility? I have to get a device to test my apps on 64bit processors, and I am interested in iPad Mini 3. However, I would like to know, if I can run apps using iPhone compatibility to see how they would work on iPhone 6 [ and iPhone 6 plus if that's possible].

Is there any way I could do it? I can't find any info about this matter, and both iPad Mini 3 and iPhone 6 share the same ppi, so the experience should be simular [accept for the fact you hold the device differently].

I cannot do the same using simulator.
Nope iPads can't run as iPhones. An iPhone app on iPad will run with compatibility mode (which is has not exactly the same behaviour as an iPhone). You'll have to test it either with a real iPhone or with the simulator.