I received a digitally signed document, is it possible to add my own digital signature and a third person's digital signature, so that this document will be signed together by three digital signatures?
As per the description above, you want to sign a digitally signed PDF file using Digital Certificates and then want another user to sign it, Is that correct?
If the owner(first user) has not locked the PDF file while signing the PDF file, then you and another user can sign the PDF file using Digital Certificates.
Navigate to Tools>Certificates>Digitally Sign and sign the PDF file.
You may also refer to Certificate-based signatures, Adobe Acrobat
Hope this helps, and let us know how it goes.