summ :: Integer -> Integer -> Integer -> Integer
summ x1 x2 x3 = x1+x2+x3

main :: IO ()
main = do 
  let x1 = 5
  let x2 = 4
  let x3 = 3
  putStr "Their Sum is: "
  putStr (show (summ x1 x2 x3))

Their Sum is: 12