diff --git a/src/jsutils/memoize3.js b/src/jsutils/memoize3.js
index 366be493fc..2d5707c5e9 100644
--- a/src/jsutils/memoize3.js
+++ b/src/jsutils/memoize3.js
@@ -3,10 +3,14 @@
 /**
  * Memoizes the provided three-argument function.
  */
-export default function memoize3<T: (a1: any, a2: any, a3: any) => any>(
-  fn: T,
-): T {
+export default function memoize3<
+  A1: { ... } | $ReadOnlyArray<mixed>,
+  A2: { ... } | $ReadOnlyArray<mixed>,
+  A3: { ... } | $ReadOnlyArray<mixed>,
+  R: mixed,
+>(fn: (A1, A2, A3) => R): (A1, A2, A3) => R {
   let cache0;
+
   function memoized(a1, a2, a3) {
     if (!cache0) {
       cache0 = new WeakMap();
@@ -33,5 +37,6 @@ export default function memoize3<T: (a1: any, a2: any, a3: any) => any>(
     cache2.set(a3, newValue);
     return newValue;
   }
-  return (memoized: any);
+
+  return memoized;
 }