I moved your post to the Metro CDMA shutdown thread
If it's like the credits in other regions you'll get a certain discount based on the phone model, regardless of how old the phone is. I believe you have to buy one of their phones though, you can't BYOD a phone and get the credit for your S3. I'd check with a store near you for what they're offering.
The ZTE Zmax is the best phone Metro currently sells, other then the high priced Galaxy S5 and S6.
If you want to BYOD a GSM phone you've got the Moto X, Nexus 5 & 6, and OnePlus One - all of which you can buy unlocked from the manufacturer, and are considered good phones.