with 3 bitcoin you can make whatever you want. for example, if i have 3 btc, i will separate 2 btc in my wallet, and use only 1 btc to trading and lending, i am sure it will grow and yes i know that for 1 btc will not grow as fast as i think but it surely grow more than i can expected. so my choice is to use for trading and lending on market exchange.
That's a good choice of investing 3 bitcoins, as with trading and lending you will make some good profits and it will also increase the number of coins in your wallet.