@Mike9241 is correct and, per Section 1.1015-4(a), your basis is the greater of what you paid your mother for the property ($3,000) or her adjusted basis at the time of the transfer.
Further, at this point, the failure to file a gift tax return (709) is largely immaterial assuming your mother passed in 2020 and did not exceed her lifetime exclusion amount of $11,580,000.
See https://www.irs.gov/pub/irs-prior/i709--2020.pdf
