when I apply for citizenship 90 days prior the 3rd anniversary of my green card (Married with US citizen) and they call you for an interview? what is the interview about? My husband needs to go also? Or is when they ask you all the questions about United States?

It is when they ask you all the US questions. From what I have read some times they require your suppose to go (for the 3 year apps) and sometimes not. They usually send you an additional letter a week or two before the interview if the spouse needs to go.

I am going to keep my dates open regardless, just in case they change their mind and I need to go.

From what I know if your I-751 is not approved, only then USCIS asks spouse to be present for Naturalization interview. During that interview, they approve both I-751 and N-400.

Spouses do not need to attend. They may ask where they are, but it's not mandatory. I never had mine with me during my interview and was just asked where she was and that was it...