The Tripartite Pact was signed by Germany, Italy, and Japan on 1940 Sep 27. By this time, Germany had already invaded Poland, conquered France, conquered Denmark and Norway, and the Battle of Britain was well underway. Italy had already invaded Albania and had begun invading Egypt, not to mention an attempted invasion of southern France (which was repulsed).
What took so long? It seems to me more sensical to formally ally, with Italy at least, before those major operations in Europe and Africa.