Язык программирования не указан. Напишу на Haskell.
import Data.List (genericLength)readInts :: Int -> IO [Int]readInts n = (fmap read) <$> mapM (\n -> getLine) [1..n]avg :: [Int] -> Doubleavg xs = realToFrac (sum xs) / genericLength xsmain :: IO ()main = do numbers <- readInts 3 print . avg $ numbers