How can I set Cortana as default assistant over Google now?

[AC Question]

Can't set cortana as default assistant over Google now

I have a Samsung Galaxy S3. I recently downloaded Cortana for android. There are multiple articles showing how to set her over Google now as default voice assistant. None seem to apply to my phone.

Is it simply not possible, or am I just doing it wrong?

If it's not possible, which Samsung Galaxy series does it start at? S4, S5?

If it's possible, please provide details of how to go about it. Because simply holding down the home button does not give me the option.

Thanks,

JPM