there are many ways to turn 3 btc into 4 btc in just 2 months but all methods are very risky. gambling is the most risky method but if you win in a high odd by chance, you will have 4 btc or even more in a very less time. btw, gambling is not recommended since you are taking risks with the money that doesnot belong to you. but you can still try it, if you can afford to repay even after loosing. other option can be trading with some altcoins. you can try ETC trading because it is very much in talk these days. btw, personally i think your friend is asking too much interest. if you really need a loan with low interest rates and have a valid collateral then you should ask for a loan in this forum and hopefully someone may help you. also why do you need this much money ? i am just curuous to know.