、。
She admitted she had lost the brooch two or three months ago.
Literal
She [topic-は] that brooch [object-を] 2-or-3 months ago lost thing-is [quotative-と] admitted.
~ものだ here works as 'it was/is the case that...', framing a fact she's acknowledging — slightly more deliberate than just 失った. The whole content of the admission gets bundled by quotative と into 認めた ('admitted, acknowledged'). 認める itself is a versatile verb: depending on context it can mean acknowledge, recognize, approve, or admit — here 'admit' fits the confessional flavor of owning up to a loss. The casual 2、3 (with the Japanese comma '、' between) is shorthand for 'two or three', expressing approximate range.