New instance declaration for Show

I'm trying to add an instance declaration in Haskell for a new data type I've created unsuccessfully. Here what I've tried so far:

data Prediction = Prediction Int Int Int
showPrediction :: Prediction -> String
showPrediction (Prediction a b c) = show a ++ "-" ++ show b ++ "-" ++ show c
instance Show (Prediction p) => showPrediction p

Seems the last line is wrong but I'm not sure how to achieve what I want. Basically is to be able to call from the interpreter a Prediction variable and get it visualized without having to call the showPrediction. Right now this works:

showPrediction (Prediction 1 2 3)

and shows:

"1-2-3"

as expected, but I would like this to work (from the interpreter):

Prediction 1 2 3

Any ideas?