or do i have to wait for an actual 10 years green card to come? I can apply for citizenship based on my married and being a green card holder for 3 years, however my question is if its sufficient to send USCIS my extension letter which is a prove of a green card?
You will have to wait until your application is approved.
Yes. I-751 must be approved. However, you can file the N-400, but it will be held up until I-751 approved.
The other lawyers are correct .. you can file now ... but it will be held up.
Talk to an attorney ... it seems like your I-751 is being held up too long.
IMMIGRATION LAW PROFESSOR for 10 years -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.