Why was it advantageous for Germany, Italy, and Japan to form the Axis alliance?

They would work together to fight against the allied powers.

Finally, on September 27, 1940, Germany, Italy, and Japan signed the Tripartite Pact, which became known as the Axis alliance. Even before the Tripartite Pact, two of the three Axis powers had initiated conflicts that would become theaters of war in World War II.