Why do people keep saying that Sakura can only use the seal every three years?
A lot of people have this misconception that Sakura can only access the Yin seal's power every three years. Not only is it illogical for Kishi to give Sakura a power up she can only utilize every three years but common sense dictates that once the seal has been formed, it is easier to maintain it.
Since the end of part one, Sakura has been slowly gathering the chakra to place inside her seal so a lot of the time that Sakura spent putting chakra into her seal was done when she was still pretty weak in comparison to now.
Since three years worth of chakra is a massive amount, it is highly unlikely Sakura can expend all of it in a short amount of time and as long as the seal remains, she can continuously put chakra into it to restore it.
If you have trouble understanding what I am saying, consider this following analogy. A car is designed, parts are gathered, put together and then sold. The car is driven, gets a bit damaged, runs out of gas and its wheels run out of air. Getting these problems fixed requires much less labour than manufacturing it thus the cost of repairing a car are usually a lot lower than its original sale price.