US citizen spouse is sponsoring me. I am using my own income on AOS case and got RFE. Responding by obtaining a joint sponsor.

The problem lies in a different matter: you are attempting to bypass professional legal assistance which resulted in a RFE in your pending case. Now, still undeterred, you are trying to complete this pending AOS case by way a few online paragraphs from attorneys who did not see any prepared form drafts in order to amend them as necessary along with all other bona fide proofs. Think what is more important for your future and made your own decision.

You are not required to submit the returns for the 2nd and 3rd most recent tax year even though you must state the income on the form. The income history may be relevant to the person's current income. Make sure to submit ample evidence of what they are earning now.

The form itself requires the last three years income, and there is an element of discretion. But most likely if the current income is sufficient USCIS would accept it.

I agree with Mr. Ivakhenko. You have posed multiple questions on this forum, seeking directeg help when you should be considering the importance and impact of completing the RFE correct and with adequate documentation to permit an approval rather than denial. Only an experienced immigration attorney can review your submitted paperwork, the RFE, the new joint sponsors papers and situation, and consider the totality of the application.