Here you can find a copy of Release Notes for Firebird 3 RC1:
http://ib-aid.com/download/docs/Firebird-3.0.0_RC1-ReleaseNotes.pdf
Firebird SQL Language Reference (for Firebird 2.5) in PDF: http://ib-aid.com/download/docs/firebird-language-reference-2.5-english.pdf
Firebird SQL Language Reference (for Firebird 2.5) in Html: http://ib-aid.com/download/docs/firebird-language-reference-2.5/