TPM really has more of a four-act structure: act one ends with the escape from Naboo, act two has the podrace as its climax, act three is the Coruscant section and act four is the return to Naboo and final battle. Four acts is unusual in most media; hour-long television dramas are the only format that regularly use it. I disagree that act one of TPM is too short; a lot happens in it. Four acts can work, and I think it does for TPM; it is also successfully used in The Hobbit. Most films have either three acts or five (2001 is an example of the latter).

AOTC has a well-defined final act, but it's difficult to say what else it has. It's very different from the other movies; until the last act, it's half detective story and half love story [insert snarky sand reference here].

ROTS has a shortish first act, and a long second one; the most likely candidate for the end of act two is Order 66.

ESB has a clear first act; it's less clear where act two ends, but I would venture Luke's departure from Dagobah makes the most sense.

ROTJ follows the standard three acts; act one ends with the sail barge battle, and act three begins with the Rebel fleet jumping to hyperspace.