No one will ever be able to tell anyone if they will be able to get 100% athletic ability back with total certainty. It depends on many factors, such as:
1. How your body copes with the process. Some people suffer greatly and develop wide legs at 2cm (I have seen it myself). Others go to 5cm with much less functional loss (I have seen it myself).
2. How old you are. If you're in your 30s you will, on average, naturally lose some athletic ability year on year. The average 34 year old will be less athletic than the average 33 year old, leg lengthening or not. Aiming for a 100% athletic recovery if you're older than 30 is quite futile.
3. Proportions before the surgery. If you have a very high T/F ration, say 88%, before surgery you can afford more femur lengthening before you fall out of the range that evolution came to consider, likely for a functional reason, desirable.
4. Complications during surgery. They can happen before you even get to 0.1cm.
5. What kind of sport you're into, as "athletic" ability can refer to so many things.
And I am sure there is more to it. Therefore there will be people who do 3cm and won't get back to 100% and then there will be some who will do 7cm and achieve the same performance levels as before the surgery, or even higher.
Having said all this, don't do it for 3cm. Yes, the 3cm are noticeable (my wife certainly noticed it when I returned to London and so did I) and yes, a good recovery is more likely than with a higher amount, but paying all the money and going for 3cm only because you want to do something like MMA? Bad idea
