Maybe: We are all the same age. OR The three of us are the same age.

You can say "We three are the same age." We three = the three of us It's not common to say "we three" but it's perfectly understandable and there's nothing wrong with saying it that way.

Of the same age works better. However, you can omit of and the meaning will remain the same. All three of us are the same age or We are of the same age

I am not sure exactly what you are trying to say. For example-- if John, Mary, and Paul are the same age (23 years old), they would say: "We are the same age"