Simplified memoize3 and improve type checking (#2964)

Author: IvanGoncharov

src/jsutils/memoize3.js

@@ -10,29 +10,28 @@ export function memoize3<
   let cache0;
 
   return function memoized(a1, a2, a3) {
-    if (!cache0) {
+    if (cache0 === undefined) {
       cache0 = new WeakMap();
     }
+
     let cache1 = cache0.get(a1);
-    let cache2;
-    if (cache1) {
-      cache2 = cache1.get(a2);
-      if (cache2) {
-        const cachedValue = cache2.get(a3);
-        if (cachedValue !== undefined) {
-          return cachedValue;
-        }
-      }
-    } else {
+    if (cache1 === undefined) {
       cache1 = new WeakMap();
       cache0.set(a1, cache1);
     }
-    if (!cache2) {
+
+    let cache2 = cache1.get(a2);
+    if (cache2 === undefined) {
       cache2 = new WeakMap();
       cache1.set(a2, cache2);
     }
-    const newValue = fn(a1, a2, a3);
-    cache2.set(a3, newValue);
-    return newValue;
+
+    let fnResult = cache2.get(a3);
+    if (fnResult === undefined) {
+      fnResult = fn(a1, a2, a3);
+      cache2.set(a3, fnResult);
+    }
+
+    return fnResult;
   };
 }