like supersnoop said -- in that situation, the 300 points used for your reservation would come back to your account. The 150 that you borrowed from 2018 cannot be banked, so they have to be used prior to June 1, 2018. The June 2017 UY points could be banked into 2018 UY.
So if you just borrow (used 2 years worth of points), then you risk losing one year's worth of points (the ones you borrowed). Similarly, if you banked 2016 points and borrowed 2018 points for that trip and then cancelled, you'd run the risk of losing 300 points if you couldn't use them prior to June 2018. This is the reason using three years worth of points can be risky -- if you have to cancel, that is a lot of points to lose.