in src/main/java/org/apache/commons/collections4/map/MultiKeyMap.java [389:397]
protected boolean isEqualKey(final AbstractHashedMap.HashEntry<MultiKey<? extends K>, V> entry,
final Object key1, final Object key2, final Object key3) {
final MultiKey<? extends K> multi = entry.getKey();
return
multi.size() == 3 &&
(key1 == multi.getKey(0) || key1 != null && key1.equals(multi.getKey(0))) &&
(key2 == multi.getKey(1) || key2 != null && key2.equals(multi.getKey(1))) &&
(key3 == multi.getKey(2) || key3 != null && key3.equals(multi.getKey(2)));
}