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] -> Double
• avg xs = realToFrac (sum xs) / genericLength xs
• main :: IO ()
• main = do
•    numbers <- readInts 3
•    print . avg $ numbers