this is gamble which gives you 33.3333% of winning your bitcoins 2x back.
Not to ruin your idea, but if you bet 33% chance on Just-Dice you get back 3x
So 3 Bitcoin returns 9 if you win, far better odds than what you are offering.
I suggest you make the bets compute very fast like Satoshi Dice or Just-Dice and make the odds far better for the user.