Your duty as estate administrator (personal representative) is to marshal the decedent's assets and prepare the requisite documents for probate and tax purposes. You should be able to locate copies of deeds, tax returns, bank and brokerage statements, et al, and proceed from that point.
With respect to liability, you can always file Form 5495 after any returns are filed on behalf of the decedent.
See https://www.irs.gov/pub/irs-pdf/f5495.pdf
With respect to the 3-year limitation, there is really nothing you can do other than filing prior to its expiration.
