I'm going to go ahead and answer anyways in case anyone else has a similar question in the future, they can reference off of this one.
Typically trade-in values are not the greatest, the only way of finding them out would be to go to the Koodo retail location and ask them or call *611 from your Koodo device.
The older the phone, the worse the value is going to be, in this case the iPhone 3 has been out for over six years now, leaving the value of it being 0. My Galaxy S4, even when it was still the best Galaxy phone (S5 didn't exist yet), the trade-in value was something like 75$. You can definitely make more money selling the phone over Kijiji or even eBay, however in your case the 3 isn't worth much anywhere you go.
Once a new version of the phone comes out, the phone you have drops dramatically in value. I do not recommend trading in at Koodo simply because you can most likely make more money elsewhere. The only benefit I could see to trading in, would be to get the money quicker.