You can only borrow points when you are actually making the reservation that will use them. (If you later cancel that reservation, the borrowed points will expire at the end of the use year into which they were borrowed).
The use year into which a reservation falls is the current use year for that reservation. For example, to pay for a for a vacation that falls within your 2016 use year, you could use any points that you previously banked into 2016 from your 2015 use year, your 2016 points and if you still need more points you could borrow them from your 2017 use year.
You can only bank one year ahead and
can only borrow from one use year ahead, so that's how "up to 3 years of points" could be available to you for one vacation. (There is a very narrow exception to the "up to three" option that could make up to 4 years available, but the actual vacation dates would have to start in one use year and continue on to end in the next use year. That very rarely happens for most people).
If you haven't already, take a look at this thread - it should help you understand a little bit better:
Understanding Use Year