Three years is about the time it takes for plants to settle in and grow a bit. But it's not long enough for most plants to get out of hand and start moving around. Even in the first seven years or so it is usually still possible to see some semblance of the original design that the gardener intended.
Actually though, I once read that most gardens need to be redone about every seven years. Unfortunately I read this useful information too late to help my own garden. It had definitely already taken the leadership role away from me by then.
I am beginning to believe that unless the gardener is a very disciplined individual, the plants are really in charge of a garden after just a few years. Look at the way they hop around and self-sow and get out of their original bounds. One day we put them here, and quite suddenly they are there.
Many plants are extraordinarily adaptable. This sometimes happens when trees die and suddenly a shade garden is in full sun. I have some vigorous and healthy masses of epimedium and hellebores that seem to love living in full sunshine, even though they are supposed to prefer growing in the shade.
Mature gardens, like mature people certainly seem to have minds of their own. The good news is that a little serendipity in a garden certainly seems to add to its charm.