So please i ask for your advice
If you are not sure on how you gonna use those 3 btc then its better to give it back to your friend and dont risk to take it and give him back with interest. You dont know how hard to grow that money, if you dont have any idea about investments then for sure you would waste all that money in no time.
Yup! you are are he is in a disadvantage in what he is doing he's gonna get 3 BTC for what? to give it back with an interest that are very impossible to allocate in just a small amount of time I think he's gonna be too deep in the red here, and you are asking in these forum for help because you don't have knowledge in raising your bitcoin I think you need to return back that money and start making your own bitcoin in a couple or methods in these forum!