You can change your phone as much as you want, it won't affect your 3 year term. The 3 year term is put in place because the iPhone originally purchased from Koodo was discounted in exchange for staying on an iPhone plan for 3 years.
If you don't want your iPhone plan anymore (maybe you lost the iPhone and you want to purchase an Android), you still haven't paid for the full cost of the iPhone. There are no cancellation penalties, you just pay the remaining cost of the iPhone. Best way to avoid all of this is to pay for the iPhone on tab or full price, by doing this you can customize your own plan! Hope that helps :)