There arent trusted ways to you to be able to make a stable income with your investment on such low time. You can try sport betting or trading some altcoins but make 0,50bt each month sound a bit hard really.
Yes sports betting can make you achieve that amount only in one day,but reverse is also the same you can also loose them all in one day so i would not recommend you this because if you loose then you will be in debt of 3btc.I advice you to return him 3btc only and start collecting or buy from your own.