How to make a Foldable Instance with multiple parameters

main.hs

import Data.Monoid

-- This code doesn't work... 
-- How can you make a multi-parameter data type be Foldable?

-- foldMap over `a` so it can be converted to a Monoid
data BinaryTree3 a v
  = Node3 a (BinaryTree3 a v) (BinaryTree3 a v)
  | Leaf3 a v
  deriving (Show)

instance Foldable (BinaryTree3 a) where
  -- foldMap :: Monoid m => (a -> m) -> t a -> m
--    foldMap :: Monoid m  => (a -> m) -> BinaryTree3 a v -> m
  foldMap f (Node3 a l r) = foldMap f l `mappend` foldMap f r
  foldMap f (Leaf3 a v) = f v
  
leaf1 = Leaf3 False (1::Int)

f1 = foldMap (\a -> Sum a) leaf1