I have a S3 phone - I want to unlock it I think?

Ok, I purchased an s3 phone about a year ago used.
1) I only use it wifi
2) I have never inserted a sim card

At this point, I realise that maybe I would need to use this phone for an emergency when there is no wifi around - so I thought I would get a SIM card from 7-11 (for example) and insert that. I don't know if the person I bought the phone from had a plan or not and I don't know if it is unlocked or not.

1) Is my logic about using a sim card for emergencies sound?
2) Will I need to unlock it to use the sim card?
3) If the above is possible- Can I continue to use wifi and only use the SIM card for emergencies?

Also I saw this for unlocking - not sure if it works or not : http://gs3.wonderhowto.com/how-to/ne...rrier-0148738/

Running Android 4.4.2
Model Number SGH-1747M
Kernel 3.4.0-178458
Canadian phone

Thanks in advance.