Best answer by rikkster
If the above fails to yield results, it's possible the SIM card may be faulty. The Galaxy S3 is an older phone and SIM cards don't last forever. With airplane mode turned off, try manually searching for a network operator, (Koodo). It's been awhile since I've played with an S3. Go to: Settings > Mobile networks > Mobile network settings > Network operators. There should be an option to manually search for a network operator.
If the S3 is unable to find the Koodo network, chances are the SIM card is faulty. The S3 takes a micro SIM card. Koodo sells a tri-cut style SIM card capable of standard, micro and nano SIM card sizes. Simply press out the size of card you need.
If your budget permits, have you thought about upgrading to a newer phone? Koodo has the Samsung Galaxy A5 (Certified pre-owned) for $450 purchased outright or on a tab small (click link for details). https://www.koodomobile.com/phones/samsung-galaxy-a5-cpo. Just a suggestion.